Dual fuel engines can use a wide variety of gaseous fuels efficiently while emitting lesser smoke and particulate than their diesel counterparts. In these engines, a primary gaseous fuel, like biogas, producer gas, LPG etc. supplies the major share of the input energy. The aim of this paper is to characterize the combustion process in a dual fuel engine and to develop a model using a combination simple law of Wiebe. The entire combustion process was divided into 3 stages as follows: the premixed combustion of diesel fuel, the premixed combustion of gaseous fuel and the diffusion combustion. The developed model can be used with good agreement to predict the rate of heat release in a dual fuel engine running at constant load and with variable diesel substitution. A fairly good agreement was observed between the simulation and the experimental results (error smaller than 2%). Copyright © 2001 Society of Automotive Engineers, Inc.