Terrain classification is necessary for the planning of timber harvesting and planning of forest accessibility. For the formation of the terrain classes, the research of terrain characteristics (terrain slope, terrain roughness index, stoniness, the risk of soil erosion by water) and stand characteristics has been used. Stand characteristics as bioclimates are spatially defined areas with a climatic type and a specific vegetation type adapted to these climates - phytocenosis and form the basis of the new terrain classification for the purpose of timber harvesting. Research of bioclimates has been proven at the level of state and private forests in the Republic of Croatia and a GIS database has been established. The conducted analyses indicate that the forest area of the Republic of Croatia can be divided into: 1. Subalpine dwarf pine forests (0.01%), 2. Subalpine spruce and beech forests (2.52%), 3. Mountainous fir, beech-fir and spruce forests (13.70%), 4. Submountainous beech forests (24.42%), 5. Sessile oak forests of hilly terrain (22.99%), 6. Common oak and floodplain forests (21.26%), 7. Sub Mediterranean mountainous beech and black pine forests (4.89%), 8. Sub-Mediterranean pubescent oak forests with hop-hornbeam (1.85%), 9. Sub-Mediterranean pubescent oak forests with oriental hornbeam (5.91%) and 10. Mediterranean Aleppo pine and holm oak forests and maquis (2.44%). Following the defined areas of bioclimate arrival and the formed GIS database of the researched area, 20 management units were selected by simple random sampling, evenly distributed in the following four economically most important bioclimates of the Republic of Croatia: 1. Common oak and floodplain forests, 2. Sessile oak forests of hilly terrain, 3. Submountainous beech forests and 4. Mountainous fir, beech-fir and spruce forests. Within the four selected bioclimates, an analysis of the degree of correction of forest roads was initially conducted. For thispurpose, the lateral deviation of each pronounced horizontal fracture of forest roads fromstraight lines and the correction factor of forest roads (ratio of straight-line length and actual length of forestroad between its startand endpoints) were studied. Research has shown that common oak and floodplain forests significantly differ from the forest roads of the other three bioclimates in the lateral deviation of the forest roads segments and the forest road correction factors. Quantity and quality analyses of the current state of forest accessibility were conducted for 20 representative management units. Since the analysis indicated poorer relative openness of almost all management units, improving the primary forest transport infrastructure was carried out by planning and designing zero line polygons to the level of very good or excellent relative openness of management units. To improve the network of primary forest roads, a model of primary forest planning was constructed and tested on the example of two management units that were insufficiently accessible. Analyses of suitable timber harvesting systems were performed using newly defined models for 20 representative state-owned management units based on their terrain and stand characteristics. Harvesting system analysis were carried out for two scenarios: 1. For the existing primary forest road network and 2. For the improved condition of forest accessibility i.e. enhanced primary forest road network. For this purpose, the following seven timber harvesting systems were selected: 1. Harvester - Forwarder, 2. Tethered harvester-Tethered forwarder, 3. Chainsaw-Forwarder, 4. Chainsaw - Tethered forwarder, 5. Chainsaw - Cable skidder, 6. Chainsaw - Small-scale cable yarder and 7. Chainsaw - Tower yarder. The results showed the possibility of using fully mechanised harvesting systems in the commercial forests of the Republic of Croatia, as well as the importance of the traditional harvesting system, Chainsaw - Cable skidder which share is particularly important in mountainous fir, beech-fir and spruce forests. In addition, the positive impact of greater forest accessibility on the share of fully mechanized timber harvesting systems is evident.