The combustion process of diesel/natural gas dual-fuel engine is complicated. In order to decouple the combustion process of natural gas (NG) and diesel, a wide range of the NG from zero to 75%, by increasing the NG flow rate at a constant air and diesel consumption rate were studied. A detailed analysis based on heat release rate data has been carried out to better understand the effect and underlying mechanism of the two fuel combustion processes. A method of calculating the ratio of heat release from NG combustion to the total heat release through comparing the heat release rate data between dual-fuel mode at different NG quantities and original diesel mode was proposed. The tested engine is a 6-cylinder turbocharged intercooler dual-fuel engine. Several combustion parameters, including in-cylinder pressure, in-cylinder temperature and heat release rate, were analyzed under 1335 rpm, 10% load (A10) condition and 1335 rpm, 25% load (A25) condition. The results show that the heat release rate curves have only one peak firstly, then transform into two peaks, and change into a quasi-single peak at last with the increase of NG quantities under A10 condition when Tinj is 7 degrees CA BTDC. The maximum ratio of heat release from combustion by NG around diesel spray to the total heat release is 25.4% and the premixed combustion of diesel occupies the main combustion process, the maximum ratio of heat release from combustion by flame propagation of NG to the total heat release is 75.4% (PES is 73%) and the combustion of NG is the main part of the combustion process. A similar discussion under A25 condition when Tinj is 7 degrees CA BTDC and about advancing and retarding Tinj under A10 condition was conducted.