Thyme (Thymus vulgaris L) is a medical and aromatic plant intensively used in pharmaceutical, food, and cosmetic industries. Our investigations carried out in 2010-2011 were focused on effect of harvest time on yield and herb quality of two thyme cultivars (‘Słoneczko’ and ‘Deutscher Winter’). Plants were harvested in the first year of cultivation in three periods: 140, 160, and 180 days after sowing (i.e. in the third decade of August, second decade of September and first decade of October). The study showed that harvesting time had a significant effect on the yield and quality of thyme. The delayed harvesting resulted in increased plant height and their mass but decreased quality of herb (lesser quantity of essential oil and thymol). The optimal time of harvest appeared to be 160 days after sowing (i.e. in the second decade of September. Harvest in this time ensured fairly good yield of herb and high its quality. The weather conditions prevailing during the vegetation period had a substantial effect on the yield and quality of herb. The ‘Deutscher Winter’ cultivar produced higher yields in favourable weather conditions, whereas the ‘Słoneczko’ cultivar exhibited a more stable yield in adverse atmospheric conditions.