To evaluate the effects of planting date and plant density on morphological characteristics, including essential oil content of ajowan (Trachyspermum ammi) a study was conducted during the years 2003 and 2004 at the experimental farm, Faculty of Agriculture, Ferdowsi University of Mashhad. Treatments were laid out in a factorial form based on a Complete Randomized Block Design of three replications. Planting was carried out in four planting dates (7 March, 28 March, 18 April and 9 May). Levels of plant density were 10, 30, 50 and 70 plants m-2 adjusted by thinning plants on rows. Results indicated that seedling emergence was affected by planting date. Delay in sowing until 9 May caused complete failure in germination. Planting date significantly affected on canopy and main stem height, number of crown, stem, primary and secondary branches, number of umbellate per umbel, 1000-seed weight as well as seed yield. The highest canopy height and the greatest number of stem branches, umbellate per umbel and seed yield were obtained in 28 March planting date and the greatest number of crown branches along with the highest 1000-seed weight were obtained in the 7 March planting date treatment. Planting density affected most of the plant characteristics. Canopy and main stem height increased with increasing in plant density. But stem diameter, number of crown, stem, primary and secondary branches, umbels per plant, seeds per umbellate, seed weight per plant, total dry matter and harvest index declined. In spite of the fact that plant density had no significant effect on number of umbellates per umbel and 1000-seed weight, mean values indicated a decreasing trend. Seed yield increased with increasing in plant density up to 50 plants m-2, but decreased when plant density was increased up to more than 50 plants m-2. Planting date did not affect essential oil percentage but significantly affected the essential oil yield. Essential oil yield decreased 34.58% by delay in planting to 18 April. Essential oil percentage and yield increased with increasing in plant density