Esterase is an important enzyme for ester hydrolysis or synthesis. Its activity, however, has not been accurately ascertained due to a lack of accurate protocols. In this study, the isosbestic point of p-nitrophenol was found and used as the marker for its activity. The methodology avoided decomposition of the substrate, chromophore agents, and pH changes. The esterase activity was determined accurately and rapidly in a complex solution. In this protocol system, organic solvents were used for dissolving substrates, which influenced activity determination to some extent. Among the solvents tested, methanol exerted the least inhibitory influence. The results indicated that this modified method has potential to be applied for esterase activity determination on a large scale and in real time.