During the last years a great deal of effort has been made for the reduction of pollutant emissions from direct injection Diesel Engines. Towards these efforts engineers have proposed various solutions, one of which is the use of gaseous fuels as a supplement for liquid diesel fuel. These engines are referred to as dual combustion engines i.e. they use conventional diesel fuel and gaseous fuel as well. The ignition of the gaseous fuel is accomplished through the liquid fuel, which is auto-ignited in the same way as in common diesel engines. One of the fuels used is natural gas, which has a relatively high auto-ignition temperature. This is extremely important since the CR of most conventional diesel engines can be maintained. In these engines the released energy is produced partially from the combustion of natural gas and from the combustion of liquid diesel fuel. The aim for the usage of dual-fuel combustion systems is mainly to reduce the particulate emissions (soot) by replacing diesel fuel with natural gas partially or entirely. For this reason in the present work are given preliminary results of a theoretical investigation using a simulation model developed for dual fuel engines. The model is a two-zone combustion one, taking into account, on a zonal basis, details of diesel fuel spray formation and the mixing with the surrounding gas, which is a mixture of air and natural gas. The main difference from a conventional diesel engine combustion system is that the natural gas is already mixed and ready for combustion. The natural gas burning initiates after the ignition of the diesel fuel and its rate depends on the rate of entrainment of surrounding gas inside the fuel jet formed. A soot model has been used to estimate the formation of soot while a detailed equilibrium model has been used to determine the concentration of chemical species. For nitric oxide the extended Zeldovich mechanism is used. The model is applied on a single cylinder test engine located at the author's laboratory at various operating conditions of the engine. The amount of liquid fuel supplemented by natural gas has been varied and its affect on engine performance and emissions has been examined. From these preliminary results it is revealed a serious effect on the heat release rate inside the engine cylinder and a reduction of particulate emissions when compared to experimental data obtained from the engine using diesel fuel only.