This paper numerically investigated the NO2 formation pathway in a natural gas (NG)-diesel dual fuel engine using a computational fluid dynamics (CFD) model CONVERGE. The fuel chemistry coupled was a reduced primary reference fuel (PRF) mechanism consisting of 45 species and 142 reactions including the NOx mechanism from GRI chemistry. The NO2 formation pathway was investigated by examining the rate of production (ROP) of key species dominating NO2 formation in each cell. The ROP of the key species was further processed to derive the representative creation reactions (RCR) of NO2 (RCRNO2). The simulation revealed that the NO2 produced during main combustion stage was formed in the hot combustion products of n-heptane spray through reaction pathway C7H15 -> CH2O -> HCO -> H -> O -> NO2 and the interface between the hot combustion products and NG-air mixture dominated by the HO2 produced through reaction pathway CH4 -> CH3 -> CH2O -> HCO -> HO2. The NO2 formed in hot combustion products during main combustion stage was later on destructed to NO and was not able to survive through the expansion process. In comparison, the NO2 formed during the post combustion expansion process was dominated by HO2 radical formed in the interface between the NO-containing combustion products and unburned NG-air mixture. It was concluded that the increased conversion from NO to NO2 in a NG-diesel dual fuel engine was due to the increased HO2 produced through the reaction path: CH4 -> CH3 -> CH2O -> HCO -> HO2 in the post combustion stage. The availability of methane necessary for the production of HO2 after the completion of the main combustion process was the main factor contributing to the significantly increased NO2 emissions from NG-diesel dual fuel engines. The research aiming at reducing NO2 emissions from dual fuel engines should focus on the approaches capable of significantly improving the combustion efficiency of NG. (C) 2017 Crown copyright and The Combustion Institute. Published by Elsevier Inc. All rights reserved.