Thymus vulgaris L. and Thymus daenensis Celak from mint family (Lamiacae) traditionally being used as source of the essential oil and phenolic components derived from their different parts. In order to optimum of harvest time this experiment has been conducted in field Islamic Azad University, Isfahan (Khorasgan) branch, in center Iran during 2012 farming season on the base of factorial experiment in the layout randomized complete block design with three replications. The aerial parts of T. daenensis and T. vulgaris were collected in five stages of plant growth, i.e. the before blooming, beginning of blooming, 50% blooming, full blooming and fruit set. The hydro-distillated oils obtained from thyme species were analyzed by GC–MS. Statistical analysis indicated that different stages of plant growth had significant effect on plant height, diameter, dry herbage, oil yield, thymol percentage and thymol yield of thyme species. Results of mean comparisons revealed that the highest fresh herbage (7639.72 kg) and the highest dry herbage (3220.55 kg) were obtained at the stage of fruit set. The highest oil content (1.62 % v/w) was obtained at the stage of beginning of blooming and the highest thymol content (69.1%) was obtained at the stage of before blooming. Comparison of interaction effects harvest time on oil content revealed that the highest oil content T. daenensis (1.41 %) was obtained at the stage of 50% blooming and the highest oil content T. vulgaris (2.42 %) was obtained at the stage of beginning of blooming and the highest thymol content T. daenensis (84.1 %) was obtained at the stage of before blooming and the highest thymol content T. vulgaris (74.8 %) was obtained at the stage of beginning of blooming. According to the results of this project at the stage fruit set for T. daenensis and at the stage 50% blooming for T. vulgaris optimum of harvest time on the quantity/quality of essential oil and thymol yield.