St. John’s Wort (Hypericum perforatum L.) has been used as a medicinal herb since ancient times, it contains several natural products with noteworthy biological activities. There is no clear information about harvesting time yield and yield components of St. John’s Wort as the plants are collected from wild. Therefore, this research aimed to determine ontogenetic (pre-flowering, full flowering and post-flowering periods) and morphogenetic (bottom, middle and top parts) variations in herb yield and hypericin content of St. John’s Wort under Diyarbakır ecological conditions during the 2004-05, 2005-06 and 2006-07 growing seasons.  Fresh and dry herb yield, dry leaf yield and hypericin content were recorded. Ontogenetic x morphogenetic interaction resulted in statistically significant effects on yield characteristics and hypericin contents. The plant was not harvested during the seedling year; whereas fresh herb yields in second and third year ranged 2721 to 5607 kg ha−1 and 2196 to 3955 kg ha−1 respectively; while dry leaf yield in the second year varied ranged 323 to 1555 kg ha−1 and in the third year 161 to 928 kg ha−1, hypericin content in the second and third year varied between 0.022 to 0.093% and 0.018 to 0.065% depending on parts of the plant. Hypericin content varied according to different parts of the plant, and the maximum value of 0.093% was obtained from the top part of the plants at the full flowering period. The results showed that there is a relationship between dry leaf yield and hypericin content of the plant parts and development stages of the plant.