A study on phytoestrogen content in Pueraria mirifica (sources) was done in the plant derived from 3 provinces of Chiangmai, Lumpoon, and Saraburi. 24 plants of each source were grown and harvested at age 6, 9, and 12 months (8 plants each). The results of analysis showed that the plants from Chiangmai, Lumpoon, and Saraburi contained oestrogenic substances of 20.56, 14.04, and 19.12 µg per 100 mg dry root powder, respectively at age 6 months and 28.12, 15.25, and 20.37 µg per 100 mg dry root powder, respectively at age 9 months. At 12 months, 30.62, 69.61, and 61.76 µg per 100 mg dry root powder, respectively were obtained. The results also indicated that P. mirifica of Lumpoon source had significantly (P<0.01) larger plant size and higher yield of tuber than those from Chiangmai and Saraburi. P. mirifica of Lumpoon and Saraburi sources had oestrogenic substances contents that were very close to those of Phrae source (63.86 µg per 100 mg dry root). P. mirifica of Chiangmai source had the least content of oestrogenic substances due to the vegetative growth of the plant before harvesting. It is concluded that the appropriate harvesting time of P. mirifica is 12 months after planting.