Nonlinear materials have been well established as photo refractive switching material. Important applications of isotropic nonlinear materials are seen in self-focusing, defocusing phenomena, switching systems, etc. The nonlinear correction term is basically responsible for the optical switches. Mach-Zehnder interferometer (MZI) is a well-known arrangement for determining the above correction term, but there are some major problems for finding out the term by MZI. We propose a new method of finding the nonlinear correction term as well as the second order nonlinear susceptibility of the materials by using a modified MZI system. This method may be used to find out the above parameters for any unknown nonlinear material.