Abstract: Soybeans are industrial crops and widely cultivated in central and northern Thailand. Isoflavones have considerable attention because of their beneficial health effects and anti-oxidant properties. The present study aimed to investigate isoflavone levels in form of aglycones (genistein and daidzein) and anti-oxidant activities among 13 soybean cultivars in central and northern Thailand. Three soybean cultivars in central region consisted of Sukhothai 1 (SK1), Sukhothai 2 (SK2) and Srisamrong 1 (SS1) and ten soybean cultivars in northern Thailand were Chiang Mai 1 (CM1), Chiang Mai 2 (CM2), Chiang Mai 3 (CM3), Chiang Mai 6(CM6), Chiang Mai 60 (CM60), SOR JOR 1 (SJ1), SOR JOR 2 (SJ2), SOR JOR 3 (SJ3), SOR JOR 4 (SJ4) and SOR JOR 5 (SJ5). Isoflavones were analysed by solid phase extraction method and anti-oxidant activities were determined using DPPH scavenging assay. The results were found that genistein was the predominant isoflavones in soybeans. Isoflavone levels and % DPPH scavenging activities among soybeans in northern cultivars were significantly higher than those in central cultivars (p=0.000). Soybean cultivars in northern region had an average of 104.08±45.25 μg g-1 for genistein, 76.78±36.91 μg g-1 for daidzein and 25.72±3.67% for DPPH scavenging activities. Soybean cultivars in central region had an average of 18.81±9.10 μg g-1 for genistein, 17.46±12.35 μg g-1 for daidzein and 14.07±1.10% for DPPH scavenging activities. The remarkable findings were that the correlation coefficient of % DPPH scavenging activities with genistein (r = 0.706, p = 0.000) was higher than those with daidzein (r = 0.497, p = 0.000). Present results therefore suggested that genistein showed the most potent anti-oxidant and high bio-availability as a promising candidate for the prevention of cancers and other diseases.