The degradation of trimethoprim(TRI) in aqueous solution was investigatecl using three kinds of low-pressure mercury lamps : UV-A (352nm), UY-C (254nm) and VUV (185nm and 254nm). The maximum absorption wavelength of the TRI at a concentration of 30mg/L in aqueous solution is 285nm with spectrophotometer. With the irracliation of UY-Iamps for 120min the degradation rates of the parent compounds showed 0 % with UV-A, 20% with UV-C and 88% with VUV, respectively. This results indicate that VUV is the most effective process for the degradation of antibiotic among LU-A, UV-C and VUV.