In the present paper the authors discuss the effect of different fuel ratios on the performance and emission levels of a common rail diesel engine supplied with natural gas and diesel oil. Dual fuel operation is characterized by a diesel pilot injection to start combustion in an intake port premixed NG/air mixture. The combined numerical – experimental study of the dual fuel diesel engine that is carried out in this paper aims at the evaluation of the CFD potential to predict the main features of this particular engine operation. The experimental investigations represent a tool for validating such a potential and for highlighting, at the same time, the major problems that arise from the actual engine operation with different NG / diesel oil fuel ratios