Much improvement has been made to the method of ninhydrin colorimetry for determination of soil protease activity. The major improvement were:introducing Ca²⁺ as protease activator, and lead acetate and sodium oxalate-acetic acid as precipitator, and using antihemorrhagic vitamin coupled with potassiumiodate to improve sensitivity and stability of the ninhydrin colorimetry in quantifying the amino acid. The buffer systems of incubation and measurement were also adjusted for better performance of the ninhydrin colorimetry. Comparison between the modified method and the widely used method of Folinphenol colorimetry was made in determination of protease activity using three different soils (Fluvo-aquic soil, yellow-brown earth and red soil) and found that the modified method is much better than the Folinphenol colorimetry for neutral and alkaline soils.