The average drag force acting on free settling particles is calculated by particles resolved direct numerical simulation. The main aim is to study and explain the effects of particle inertia on drag force in free settling particles for different solid volume fractions and Reynolds number. Immersed boundary method (IBM) is used for particle fluid interactions and discrete element method (DEM) is used for particle-particle interactions. The particle inertia is varied by changing the density ratio of particles to fluid and keeping the particle diameter constant. In simulations, particle to fluid density ratios equal to 2, 250, 1000 and 2500 are used. For each density ratio, Reynolds number and solid volume fraction are varied from 0.1 to 400 and 0.005 to 0.2 respectively. it is observed that the drag force acting on particles with larger inertia or particles with longer response times is similar to that of fixed particles. As the particle inertia decreases or particles with smaller response times, the drag force on free settling particles increases from the fixed arrangement of particles. The only exceptions are the settling of particles in dilute suspensions and for Reynolds number more than 200 in which the drag force becomes smaller than the drag force for fixed arrangement of particles. It is observed that this behavior of settling particles is either due to the formation of particle microstructures or velocity fluctuations. At the end of paper, an improved drag correlation is proposed which can be capable of accurately calculating the drag force in Eulerian-Eulerian and Eulerian-Lagrangian simulations for particles with different densities. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.