This paper studies the influence of Soil-Structure Interaction (SSI) on the seismic response of the base isolated Reactor Building (RB) of the Pressurized Heavy Water Reactor (PHWR) resting on the hard rock founding media. 1.6 m diameter elastomeric rubber bearing with a lead core (ERBL) are selected for seismic isolation of RB. SSI analysis of RB is performed using 0.36 g peak ground acceleration severity earthquake level which corresponds to the Beyond Design Basis Earthquake Event (BDBE) for a representative Indian Nuclear Power Plant (NPP) founded on rigid rock. The nonlinear behavior of the base isolator is modeled using the constant effective stiffness based on the design displacement of the ERBL isolator. The dynamic response of seismically isolated RB using standard and sub-structuring approaches of SSI analysis is evaluated. Seismic responses in terms of accelerations at key locations in RB structure are studied thoroughly. The results of this research show that the application of seismic isolation works effectively in reducing the structural frequencies and floor accelerations to a greater extent than the non-isolated condition. The standard approach of assuming fixed base condition and SSI analysis using sub-structuring approach for base isolated RB founded on rigid founding media and by using constant effective stiffness of the isolator shows a minimal variation in the floor accelerations. Hence, it is concluded that the SSI effect for base isolated structures founded on rigid founding media is very minimal, however seismic motion incoherency, varying founding media and incorporation of nonlinear behavior of isolator during analysis needs to be studied to evaluate the SSI effect on the seismic response of base isolated RB.