In order to explore the effect of the flow rate of Ar on the properties of diamond-like carbon (DLC) films. The DLC (a- C:H) films are prepared on Si substrate by RF- PECVD technology with n- butane(C4H10) as the reactant, the effect of Ar on the properties of DLC films is studied by changing the flow rate of Ar. Determination of Fourier transform infrared spectroscopy, hardness, thermal stability and roughness of the surface demonstrates that the relative content of H in the DLC and the hardness of DLC films decrease with the increase of the flow rate of Ar, the hardness decreases from 23 GPa to 20 GPa, and the roughness of the surface increases from Ra= 3.732 nm to Ra= 8.628 nm, the flow rate of Ar almost has no effect on the thermal stability of DLC films, but the stress decreases from -1.8 GPa to -1.1 GPa.