Abstract: In order to evaluate resistance of Commelina commumis L.biotypes to fomesafen whole-plant experiment and protoporphyrinogen oxidase（PPO）activity，determination was carried out by greenhouse pot method.Results showed that biotype of Commelina commumis L.from collecting sites typical black soil region of Heilongjiang province developed different levels of resistance to fomesafen and the highest resistance index（RI）was 2.98 which was found in biotypes from Hailun city.The IC50 value of fomesafen against PPO in Commelina commumis L.from Hailun city was 1.73 times comparing with S biotype in vitro.Results suggested that decreased PPO sensitivity may be one of main reasons that responsible for the resistance of Commelina commumis L.biotypes to fomesafen.