Ammonia plasma treatment is an efficient method to modify the surface of polymeric biomaterials to improve their hydrophilicity and biocompatibility. In this study, poly(l-lactic acid) (PLLA) films were treated with ammonia plasma to investigate the effects on the surface properties and cytocompatibility. Surface morphologies of the films were observed with atomic force microscopy (AFM) and the surface roughness was analyzed with the software attached to the AFM. Mass loss density and contact angles associated to plasma treatment power and time were also studied. The stability of the treated films was evaluated by testing the contact angle change. The cytocompatibiliy was evaluated by cell adhesion, proliferation and cell cycle. The results showed that the surface morphology and roughness of the treated PLLA surfaces increased with treatment power. The mass loss density increased with plasma treatment power and time. With increasing treatment power and time, the resulting amino group density on treated PLLA film surface increased first and decreased later, while the contact angle showed an opposite trend. The contact angle of the treated films increased with storage time and returned to its original value after about 2 weeks. The cell experiments indicated that promotion of cell adhesion and proliferation were significantly improved on the treated PLLA surfaces.