This paper presents a numerical study of air filters used for an off-highway vehicle by using commercial CFD code FLUENT v 6.1. For the primary filter, the flow is obtained by the use of Reynolds stress model and stochastic Lagrangian model is used to study the gas solid flow. The separation efficiency for dust particles of sizes 50 and 100 microns is found to be 94.4 %. For the secondary air filter, filter portion is modeled as equivalent porous medium. The pressure drops obtained by the analysis agrees very well with the experimental results. The convective scheme is found to have little effect on the prediction of pressure drop through the secondary filter.