Abstract: BJECTIVE To establish a method for the simultaneous determination of ethyl hydroxybenzoate, thiomersal and benzalkonium chloride in diclofenac sodium eye drops by HPLC. METHODS The C₁₈ column was used as analytical column, with 1% triethylamine solution(phosphoric acid adjusted pH to 3.0) as mobile phase A, and methanol as mobile phase B with gradient elution. The column temperature was kept at 40℃. The flow rate was 1.0 mL·min^-1. The detection wavelength was set at 254 nm. RESULTS Between 20.58-205.8 μg·mL^-1(ethyl hydroxybenzoate), 8.424-82.42 μg·mL^-1(thiomersal), 12.88-128.8 μg·mL^-1(benzalkonium chloride n-C₁₂H₂₅), 6.624-66.24 μg·mL^-1(benzalkonium chloride n-C₁₄H₂₉), their linearities were good, with their recoveries were between 99.3%-102.5%(n=9). CONCLUSION The method is simple, accurate and reproducible. It can be used to control the content of ethyl hydroxybenzoate, thiomersal and benzalkonium chloride in diclofenac sodium eye drops.