Impacts of pH, temperature and coexisted proteins on degradation kinetics of two flavonoids fisetin and quercetin were assessed by spectroscopic method in the present study. Based on their degradation rate constants (k), fisetin was more stable than quercetin. Increasing medium pH from 6.0 to 7.5 at 37 °C would enhance respective k values of fisetin and quercetin from 8.30*10−3 and 2.81*10−2 to 0.202 and 0.375 h-1 (P<0.05). In comparison with their degradation at 37 °C, fisetin and quercetin at higher temperature showed larger respective k values (0.124 and 0.245 h−1 at 50 °C, or 0.490 and 1.42 h−1 at 65 °C). Four protein products in medium provided stabilization to both fisetin and quercetin (P<0.05), as protein addition at 0.10 g L-1 could decrease respective k values to 2.28*10−2-2.98*10−2 and 4.37*10−2-5.97*10−2 h−1. Hydrophobic interaction between the proteins and the two flavonoids was evidenced responsible for the stabilization mostly, as sodium dodecyl sulfate could destroy the stabilization significantly (P<0.05). Both casein and soybean protein products provided greater stabilization than whey protein isolate. It thus concluded that higher temperature and alkaline pH will bring about greater flavonoid loss, but proteins can inhibit flavonoid degradation.