ZrN thin films were prepared on 45# steel by using ECR-microwave plasma source enhanced dc magnetron sputtering. The increase of the N2 partial pressure PN2 leads to phase transitions from ZrN and an orthorhombic ZrNx phase to an amorphous phase. At PN2 = 1.1 × 10-2Pa to 4.5 × 10-2Pa, the films contain both ZrN and ZrNx (a = 0.3585, b = 0.4443, c = 0.5798 nm). At PN2 = 5.0 × 10-2 Pa, the film shows a strong tendency towards the amorphous phase. The N concentration at different PN2 varies from 7.73 to 66.96%o according to electron probe analysis. The microhardness of the samples, varying from 19.82 Gpa to 26.73 Gpa, first increases and then decreases with increasing PN2. The hardest sample has a wear rate of about 4.5 × 10-5 mg/min. The property changes are relative to the film structure due to different N2 partial pressures.