The major advantage of using dual-fuel technology is diesel-like efficiency and brake mean effective pressure (BMEP) with much lower emissions of oxides of nitrogen (NOx) and particulate matter. Because of these, many research has been done on dual fuel technology. In this paper, calculations are done to determine the stoichiometric air to fuel ratio of the natural gas, diesel and both of them as a mixture. The results is used in modeling and simulating the air-natural gas mixer using computational fluid dynamic (CFD) software called CFD-ACE+ available in the university. The mixer is a simple device that mixes the natural gas and air at the air inlet prior to entry into the combustion chamber. It will promote natural gas and air to mix homogeneously. The mixer will be fabricated based on the best simulation results.