The purpose of this work was conducted to determine the various contents of isoflavone from soybean [Glycine max (L.) Merr.] leaves under different development periods. Three representative isoflavones including daidzein (1), glycitein (2), and genistein (3) were isolated from soybean leaves and identified using high performance liquid chromatography (HPLC)-diode array detection (DAD) and HPLC-DAD-ESI/MS. Total isoflavone contents increased with increasing growth period. No significant differences were observed in total average contents between 4 June 2007 and 4 July 2007 sowing dates, whereas significant differences were observed between 15 May and 25 May sowing dates (total average contents: 210.00${pm}$3.19, 15 May and 145.56${pm}$2.52 ${mu}g/g$, 25 May). Saeolkong cultivar showed the most significant changes between 15 May (313.29${pm}$3.47 ${mu}g/g$) and 25 May (175.19${pm}$1.71 ${mu}g/g$) sowing dates. The highest amount of total isoflavone was found in Saeolkong (313.29${pm}$3.47 ${mu}g/g$, 15 May), whereas Cheongja 3 showed the lowest amount (88.59${pm}$1.50, 4 July). Genistein was the predominant isoflavone, followed by daidzein and glycitein in 48 samples of soybean leaves. These results showed that isoflavone content and growth period of soybean leaves have positive correlation, which could be a key factor in the selection process of a high quality species.