Additive manufacturing (better known as 3D-printing) is a technology which enables creating a wanted object by adding the material layer by layer. One of the most wide-spreaded technologies of additive manufacturing is fused filament fabrication (FFF). FFF is a technology in which the deposited polymer material filaments are applied to the working surface, layer by layer, by means of a mobile nozzle. In order for the technology to function and to obtain a quality finished product, it is very important to prepare the filaments of an appropriate diameter. Filaments which are used during the process ought to be 1.75 mm or 3 mm in diameter. In this paper the process of cycloolefin copolymer (COC) filaments extrusion was optimized. The experiment was carried out in Noztek Pro single screwed extruder. COC exhibits unique combination of properties such as: high transparency, great water vapour barrier properties, excellent resistance to water and other polar solvents, and biocompatibility. It is a material with good mechanical properties which makes it appropriate for Fused Filament Fabrication. The optimization was carried out in the Design-Expert computer program and optimal conditions were achieved for obtaining an ideal diameter of 1.75 mm and a minimum standard deviation. Ten different combinations of desired conditions were found in which the desired diameter values and small standard deviations were obtained. Combination with the highest desirability gives an ideal diameter of 1.75 mm with a minimum standard deviation of 0.06 mm, and it is achieved at a temperature of 197.93 °C, puller speed of 51.22 mm/s and puller distance of 44.43 cm. In this paper was also carried out the fabrication of COC composites. They were fabricated using functional fillers: precipitated calcium carbonate and calcium carbonate pretreated with stearic acid. The mentioned composites were fabricated in order to research the possibility to modify mechanical, thermal, surface properties and resistance to solvents (water, acetone and ethanol). By carried out contact angle analysis it is determined that the functional fillers do not affect the hidrofobic surface properties of the composites and it is established that there were no significant modifications in terms of mechanical and thermal properties. In addition, the fabricated composites and COC itself have shown excellent resistance to water, acetone and ethanol.