Construction and applications of synchrotron light sources are developing rapidly. Achievement to smaller emittances is one of the most important issues in the design and construction of fourth generation synchrotron light sources. Parameters of the stored electron beam such as emittance and beam instabilities are affected with many different phenomena. One of the most important components of the storage rings which have great influence on the beam instabilities and emittance are radio frequency cavities. By reducing the operating frequency of the RF cavity, it is possible to achieve lower emittances and more favorable parameters for synchrotron operation.This presentation examines the design and simulation of a 100 MHz cavity for operation in fourth generation low emittance synchrotron light sources. The prototype of this cavity is under fabrication based on presented simulation results and will be used for operation in the Iranian light source facility (ILSF).