This paper discusses about the effect of the porosity on the heat transfer and fluid flow phenomena in Pebble Bed Modular Reactors (PBMR) core. Computational fluid dynamics (CFD) code has been employed to simulate the unsteady state thermal – fluid phenomena in the PBMR core, in which helium is used as the coolant. In this work, the ratio of coolant mass flow rate to the fuel volume was assumed constant. The outer wall of the core was kept at a constant temperature, while the top and bottom walls were assumed to be adiabatic. In this paper, the effects of porosity on the thermo-fluid-dynamic phenomena in the PBMR core has been investigated. Results show that porosity has a significant influence on the pressure drop, the reactor power, the coolant mass flow rate, the coolant temperature and the normalized power.