A pulsed green laser with wavelength of 532 nm is employed to scribe the sapphire substrate. Firstly, sapphire surface ablated by single pulsed laser is observed. The ablation threshold and the cracks threshold of sapphire are identified. The relationship between the laser fluence and crater depth/diameter is analyzed. Then, the thermal stress of nanosecond green laser irradiating sapphire is analyzed. Finally, the parametric laser scribing of sapphire substrate is conducted. The results show that the formation of the craters and grooves in the laser ablation process is mainly due to the photothermal effect. The effect results in melting, vaporization, which is essential to the material removal. Meanwhile, micro-cracks are easily induced by the larger thermal stress. A narrow groove width of 20 μm, aspect ratio of 7 and better processing quality can be obtained simultaneously by considering laser processing parameters (laser fluence, scanning velocity and scanning times) and the treatment of the material surface.