Surface hardening treatment of titanium materials in use of CO gas was studied including investigation of post heat treatment under vacuum. C.P. titanium, α+β type SP-700 alloy with Ti-4.5%Al-3%V-2%Mo-2%Fe and β type alloy with Ti-15%Mo-5%Zr-3%Al were used. Surface hardening was conducted by heating these materials at 1073K for 21.6ks in Ar-5%CO gas. Subsequently, specimens subjected to surface hardening were heated at 1073k for various time periods under vacuum. While the maximum surface hardness value was the largest in C.P. titanium and the least in SP-700 alloy, hardening layer thickness was the thickest in β type alloy and the thinnest in C.P. titanium. Surface hardening in C.P. titanium was brought about by solid solution hardening due to oxygen and carbon. Enrichment of these elements in the surface layer of both titanium alloys caused continuous variations of the microstructure such as β to a+β, or their volume fractions in the surface hardening layer. Post heat treatment at 1073K increased the maximum surface hardness and hardening layer thickness with an extension of the heating time in C.P. titanium, but the surface maximum hardness decreased continuously in β type titanium alloy. Post heat treatment could remove the thin oxide layer formed by surface hardening treatment.