In this work, a thermal-hydraulic investigation of N₂ as a coolant in a pebble bed reactor core has been performed using a porous media approach. Three different diameters of fuel sphere have been employed for the numerical simulations. The pebble bed reactor is a kind of packed bed reactor whose core is a long right circular cylinder with a height of 3.5 m and an outer diameter of 3.7 m. The finite volume method was used to solve the governing equations using ANSYS FLUENT 14.5. Several important thermal-hydraulic parameters have been investigated consisting of the coolant and solid temperatures, density, pressure drop, and the coolant temperature. Results show that when the fuel sphere diameter increases, the coolant temperature is almost the same, but the solid temperature increases and the pressure drop decreases.