Performance of a nano-scale MOS (metal-oxide-semiconductor) can be significantly improved by uniaxial stress, caused by the SiN film deposited on the surface of MOS. Although this technique has been widely used in the performance improvement of CMOS and integrated circuit, the physical mechanism for instance, how is the strain in MOS channel caused by the SiN film? how about the relation between the kinds of the structure of SiN film needed to be discussed in depth. On the basis of the ISE TCAD, three typical models for stress analysis——such as the segmentation structure model, the closed-loop structure model and the integrity structure model——are proposed. And then, this paper reveals the physical mechanism about how the stress in MOS channel is caused by the SiN film and how much the magnitude of the stress in MOS channel is induced. Results shows that: 1) The “step” structure is the necessary condition for the strain in the MOS channel to be caused by the SiN film. 2) With the tendency for SiN film to shrink or expand, the film may lead to the deformation along the MOS source/drain region of the Si material, which causes the deformation of Si in the channel. 3) The whole of the channel stress in SiN film is equal to the sum of the stress in the source/drain imposed by the SiN film above the source/drain, the stress which the “closed loop structure” applies to the channel, and the stress generated in the channel by the whole SiN film. Our conclusions may provide the valuable references to the manufacture of nano-scaled MOS and the research of the novel inducing stress technique.