Rosemary (Rosmarinus officinalis) is an evergreen perrenial shrub that is used in pharmaceutical and food processing industry. The most commonly used is rosemary leaf, while the flower and green rosemary branches are rarely used. An attractive smell and rosemary taste are some of the characteristics used in the pharmaceutical and food industry. Apart from the fresh state, rosemary leaves can be thermally processed to preserve qualitative properties. For this reason, it is of great importance during the thermal treatment process to determine the optimal combination of temperature and drying time to keep the value of dehydrated sheets at the highest quality level. For the purposes of this study, rosemary leaves were harvested in winter and spring harvest. The aforementioned herbal material was collected in the area of Istria where rosemary in the wild grows as self-propagating herb. The leaves were dried at two different temperatures (400C and 500C) and in the air. Their chemical properties were determined before and after convection and natural drying. The research proves that rosemary leaves harvested in spring had the fastest drying rate dried at air temperature. The total ash, fat content and the share of C, H, N, S were determined on fresh and dried rosemary leaves.