WTaN hard films with Ta/(W+Ta)=46at.% were deposited on single crystal Si (111) substrates using direct current magnetron sputtering. The effect of nitrogen partial pressure (pN2) on crystal structure, surface topography, adhesion strength, and hardness of WTaN films was investigated. With increasing pN2, the phase composition changes from pure fcc W-Ta-N phase to a mixture of fcc WTaN phase and hexagonal δ-W(Ta)N phase, and then to pure hexagonal δ-W(Ta)N phase; the average grain size decreases monotonously; the surface becomes more and more smooth; the hardness initially increases and then decreases after passing a maximum of 41GPa at pN2=0.5Pa, while the adhesion strength varies in an opposite trend to the hardness. The maximum hardness could be due to the combined effect of reduced crystallite size and the coexistence of two phases.