In order to improve the hardness of crew thread of titanium alloys by forming a uniform film, the effects of plasma nitriding treatment on microstructure and performances are studied by metallographic structure, hardness, strength etc. The results show that with the increase of temperature from 700℃ to 850℃, the tensile strength decreases from 1 026 MPa to 930 MPa, and the elongation increased, however, the hardness improves greatly from 529 HV to 826 HV compared with the untreated specimens(280 HV). The nitriding time has little effects on microstructure and hardness. A uniform film is fabricated by using these parameters on TC4 titanium alloy. The improvement of hardness mainly is caused by the formation of Ti₂N and α-Ti co-film.