We simulated stress components in three directions in the Si channel of NMOSFETs with SiC alloy S/D stressor and tensile CESL in this study. The resulting saturation drain current enhancement was analyzed. Tensile stress along the transport direction was found to dominate mobility enhancement. Stress along the width direction was found to affect drain current the least. However, for NMOSFETs, the compressive stress along vertical direction perpendicular to the gate oxide the makes considerable contribution to mobility enhancement and can not be neglected. To obtain the new mobility model for this study, we extended a simple model for strain effect in Si band, to include both shear strain and the quantum confinement effect in the inversion layer of NMOSFETs.