Surface oxygen diffusion hardening treatments of TC4 titanium alloy in Ar+O2, O2 and ambient atmosphere were performed by scanning with a pulsed Nd: YAG laser. By using different gas atmosphere with Ar+O2, O2 and ambient atmosphere, the structure and the hardness behavior of the laser oxygen hardening layers can be controlled in a wide range. Optical microscope (OM), scanning electron microscopy (SEM), energy dispersive analysis system of X-ray (EDAX) was used to reveal the microstructure and to identify the chemical composition. A Vickers microhardness tester was employed to measure the hardness distribution across both the substrate and the hardening layer. It is shown that smooth, deep and crack-free harden layers of a surface hardness ranging from 810-673Hv can be obtained under ambient atmosphere and Ar+O2 atmosphere. Cracks are observed in the sample processed under pure oxygen atmosphere. Dilution of oxygen gas with argon or nitrogen (ambient atmosphere) leads to a crack-free harden layer at the expense of a reduction in surface hardness.