The RP-HPLC method was used to determine the 5-aminosalicylic acid (5-ASA) in mesalazine enteric-coated tablets.5-ASA was derivatized to acetyl 5-ASA, RP-HPLC was adopted with a ODS C18 column, a mobile phase consisting of phosphate buffer-acetonitrile(9∶28) at 0.9 mL·min-1,and the UV detective wavelength was 304 nm.The calibration curve of 5-ASA was linear in the concentration range of 3.12~50.0 μg·mL-1,the average recovery was 100.8%  with RSD of 1.27% . The method was credible, and specific the operatin was simple and the result was accurate. The method was applicable to determine the 5-ASA in mesalazine enteric-coated tablets.