Effects of low molecular weight organic acids on phosphomonoesterase activity in red soil were investigated in lab. Results show that phosphomonoesterase activity was enhanced by organic acids in low concentration (< 1 μmol g^-1), and the effect followed the order of citric acid ≈ oxalic acid > malic acid > tartaric acid, but inhibited by organic acids in high concentration (> 5 μmol g^-1) and the effect followed the order of citric acid > oxalic acid > malic acid > tartaric acid. On the one hand, phosphomonoesterase activity was improved when pH of the system approached to the value, where enzymic reaction was optimal, while inhibited when the pH varied the other way. On the other hand, phosphomonoesterase activity was increased by organic acidic radical via carboxyl, while inhibited by release of metal ions.