Laser-induced damage in optical film remains a great challenge in high power laser systems. The mechanism of inclusion damage to film had been studied: firstly, the film damage morphologies had been observed and then the effects of inclusions on film had been analyzed. The research results show that the damage effects of inclusions on film can be divided into thermodynamic effects, interference effects due to scattering and laser plasma effects, the combined action of those damage effects determine the characteristics of damage points. The effects above are closely related to the particles radius: for smaller radius, the deposited laser energy is less and temperature is lower, as the heat diffusion range is smaller and damage points are mainly due to the melting. While for larger radius, the energy deposition and temperature are higher, and the laser plasma can be formed easily and cause big ablated pits.