A Q-switched Nd:YAG laser is used to fabricate micro channels in a fused silica substrate by laser-induced plasma processing. The interior wall of the micro channels drilled by laser-induced plasma is smooth, and the depth of the channel is up to 4 mm. High-power laser beam focused on the fused silica can lead to optical breakdown, resulting from the formation of plasma. The ablation of the high temperature plasma can lead to a micro channel. We study the ionization mechanism of optical breakdown in solids by nanosecond laser pulses. The changes of optical parameters, including dielectric function, reflectivity and refraction index, are studied. We also discuss the critical free electron density of optical breakdown and calculate the range of optical breakdown.