Abstract: The resistance to Phoxim in Panonychus citri was selected in laboratory. After 12 generations of selections, Panonychus citri have developed 18.6-fold resistance to phoxim. By the Comparison of the properties of mixed function oxidase (MFO) between the resistant and susceptible strains and the inhibition of phoxim and PBO to MFO of P. citri, it was found that the activities, specific activity and the Michaelis–Menten constant (Km) of MFO in the resistant strain were significantly higher than those in susceptible strain, and the inhibition of phoxim to MFO in susceptible strain was significantly greater than that in resistant strain. There was a significantly linear relationship between the inhibition rate of MFO activity and the inhibition concentration of the Phoxim. However, the inhibition of PBO to MFO in resistant strain was significantly higher than that in susceptible strain. It was suggested that the development of resistance to phoxim in P. citri might be related to the rise of the MFO activity.