The change of conformation and activity of human placental alkaline phosphatase (PLAP, E.C.3.1.3.1) in pentachlorophenol (PCP) solutions of different concentrations were studied by enzymic activity measurement and fluorescence spectra. The inhibition type of PLAP by PCP and the effect of pH on the enzyme inhibition were simultaneously measured. In the PCP concentrations lower than 1.0 mmol/L, the enzyme activity and fluorescence intensity rapidly decreases with a marked red shift of emission maximum. In the PCP concentrations higher than 1.0 mmol/L, the enzyme activity and fluorescence intensity gradually decreases with a continuous red shift of emission maximum.At 5.0 mmol/L PCP, the enzyme intrinsic fluorescence quenched and the enzyme activity is 51.4% of original activity. At 10.0 mmol/L PCP, the enzyme residual activity is 30.0%. PCP is an uncompetitive inhibitor of PLAP. The inhibition constant(K_i) is 3.86 mmol/L.The activity inhibition of PLAP also affected by the pH, its activity inhibition disappeared below pH 7.5,but gradually increasing between pH 7.5～10.5. The results suggest that the activity inhibition and the conformation changes of the enzyme were caused by PCP. The inhibition of PLAP activity correlates with the dissociation state of PCP.