Kouhi thyme is belonged to Lamiaceae family and widely used in pharmacy, cosmetics and Food industries of developed countries; it is a strong disinfectant with antioxidant. Study about the effects of environmental conditions such as harvesting times (phonological stages) on herbage yield and quantity/quality of essential oil is very important goal to obtain maximum productivity of this valuable medicinal plant. Therefore, in order to determine the effects of phenological stages on herbage yield and quality/quantity of oil in Kouhi thyme (Thymus kotschyanus Boiss. & Hohen.), this study was conducted in research farm of Khorasgan (Isfahan) branch of Islamic Azad University in 2009-2010 years using a randomized complete blocks design with three replications. Plants were harvested in five phenological stages, i.e. in the vegetative stage, beginning of blooming, 50% blooming, full blooming and fruit set stages. Oils were extracted by hydrodistillation for 3 h, of the aerial parts using Clevenger-type apparatus. Thymol percentage was determined by GC/MS. Results showed the significant effects of phenological stages (harvesting times) on fresh and dry herbage yield, essence and thymol yield and percentage. Mean comparison showed that 50% blooming has the highest essence percentage (1.48%). According to GC/MS analysis, thymol was the main compound of oil in all-phenological stages. The highest fresh and dry herbage as well as essence and thymol yields were obtained from fruit set stage. In conclusion, it is recommended to harvest this plant in fruit set stage to gain the highest essence yield, thymol yield and fresh and dry herbage.