Laser assisted machining (LAM) is effective for machining brittle and hard materials such as engineering ceramics. Because it has larger technological space, is more economical, and can achieve a better surface integrity. In this paper, two sets of singlefactor experiments were performed in which the laser power and the cutting depth were changed respectively. The change of the surface roughness under different material removal modes for LAM of Si3N4 was studied using a variety of parameters. The results show that the values of Ra and Rq are smaller in the plastically machining state than that obtained when the material is removed through a brittle fracturing or thermal damage occurs. When there is no thermal damage, the value of Ra and Rq decrease with the increase of the softening degree of the workpiece. In the ductile material removal mode, the distribution of height amplitude curve of the surface tends to be more symmetrical and more normally; the surface contour is more fine.