The mutual interaction of daidzein with bovine serum albumin (BSA) was investigated by means of fluorescence spectroscopy. The results revealed that daidzein caused the fluorescence quenching of BSA through a static quenching procedure. The value of binding constant (Ka) were 1.17 × 104 M−1 (T=310K), 3.74 × 104 M−1 (T=303K), 5.36 × 104 M−1 (T=298K) respectively. Thermodynamic parameters ΔG0, ΔH0, ΔS0 were calculated, besides the van der Waals force and hydrogen bonds play a significant role in spontaneous stabilizing the complex. Moreover, the effect of Cu2+, Ni2+, Mn2+ and Co2+ on the binding constants between daidzein and BSA were studied.