The chemical properties as well as the mechanical properties of a ternary (Ta, Si)N hard coating film on high-speed tool steel were investigated. Deposition of the film was made by a reactive sputtering method with the flux of N-2 as the major variable. From Auger electron spectroscopy it was confirmed that the deposited film was composed of Ta, N and Si with the content of N increasing with the N-2 flux during deposition. In as-deposited condition, the major phase of the film, as analyzed by an X-ray diffraction, was TaN that contained Si in solution. When post-coating annealing heat treatment was conducted, the chemical composition of the coating was changed due to an outward diffusion of C and Fe from the substrate, which was most significant at 1000 degrees C. The micro-hardness of the as-deposited coating increased with the flux of N-2, reaching to 17.1 GPa at 16 seem, and this increased to 29.8 GPa after the 1000 degrees C annealing heat treatment. The critical load in the adhesion test was also increased by the heat treatment: from 12 N in as-deposited condition to 33 N in the heat-treated condition. The improved mechanical properties in the heat-treated samples were attributed to TaC formation in the coated film and enrichment of Fe and Si at the interface of the coating and the substrate. The role of Si in the (Ta, Si)N coating was two-fold: stabilizing the amorphous structure up to 900 degrees C and enhancing the mechanical properties such as hardness and adhesion strength upon high temperature exposure of the coated film.