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Increasing the current, beam lifetime and, the quality of dynamic properties of the stored electron beam in the storage ring are the important purposes of synchrotrons. One of the phenomena affecting the beam quality is the longitudinal and transverse instabilities that increase as the beam current increases. Longitudinal instability in radiofrequency cavities has the greatest impact due to the high-quality factor of their higher order modes. As a result of these modes, one electron bunch affects the next one and it has a long-range nature. Longitudinal instabilities increase the energy spread and lead to loss of the beam current and reduce the intensity of the output synchrotron radiation. Considering the current and emittance of the Iranian light source which is 400 mA and 270 picometer-radian in the final phase of project, it is important to evaluate the higher order modes and identify the most effective ones. Simulation studies using CST Studio Suit software show that there are 13 monopole modes up to 1500 MHz in the project's 100 MHz radio frequency cavity. Seven modes are dangerous, and two modes at frequencies of 624.67 and 1432.55 MHz, with shunt impedances of 70.8 and 115 kHz, are very effective and must be dealt with in all phases of commissioning.