Abstract: To study the measurement of serum paraoxonase activity which is of significance in the some cardiovascular disease and intoxication of organophosphates,using a simple spectrophotometer.Methods The liberation of P-nitrophenol upon hydrolysis of paraoxon at 412nm was measured to represent the enzyme activity.The reactive conditions were optimized and the final assay system consisted of 10μl serum added 1.6ml mixture of 1.0mM paraoxon and 1.0mM CaCl₂ in 0.05M glycine buffer(pH10.5),incubated at 25℃ for 10 minutes and inhibited by 50μl 0.1M EDTA.Results The paraoxonase activity was linear correlated with the absorbance value between 0 and 1600 units.The coefficient of variation(CV)of this met hod was from 1.49%,to 4.27%.The difference between observable and theoretical value was 17.11%,1l.2% and 4.95%,respectively,by adding 25%,50% and 75% of the enzyme(in serum).The CV among different daily measurement varied between 10.11% to 10.54%.Conclusions The method is good enough to detect the paraoxonase activity and can be used widely,since it needs simply instrument.