As the potentials of experimental studies are still limited, a predictive resist image simulation of Immersion lithography is very important for a better understanding of the technology. One of the most critical issues in Immersion lithography is the description of the influence of immersion which is the presence of a uniform liquid layer between the last objective lens and the photo resist, on optical lithography. It enables the real part of the index of refraction in the image space, and the numerical aperture of the projection lens, to be greater than unity. Therefore, it is virtually involves Maxwell vector solution approach, including polarization effects and arbitrary thin film multi-layers. This paper discusses the improvement in process window afforded by immersion under a variety of conditions, including 193nm and 157nm, Off-axis illumination, Attenuated Phase Shift Mask for 65nm and 45nm technology node. Comparisons with dry and liquid lithography simulations are used to evaluate the availability and the performance of the proposed approach. The implemented resist simulation approach is examined the impact to the process window of variations in liquid refractive index as well.