The oxidation behavior of three types of plasma-enhanced chemical vapor deposition (PECVD) processed Ti–Si–C–N coatings with silicon content ranging from 4.3 to 11.6 at.% has been investigated at high temperatures. Systematic characterization was conducted to study the evolution of composition, phase constituents, hardness, surface morphologies, microstructures, and grain size during oxidation. A two-stage oxidation process was observed between 700 and 1000 °C for all three coatings. Experimental results indicate that a superhardness of 40 GPa can be maintained up to 700, 800, and 850 °C for 4.3, 7.4, and 11.6 at.% Si coatings, respectively; the dual-phased 7.4 and 11.6 at.% Si coatings show a better oxidation resistance than the single-phased 4.3 at.% Si coating. On the basis of the results, a mechanism is proposed to explain the relationship between the nanostructure and oxidation behavior.