<p>Dual fuel engine combustion characteristic using the mixture of compressed natural gas (NG) and ethanol as low reactivity fuel and diesel as high reactivity fuel was investigated experimentally in this work. Experiments were performed on an optical engine with double diesel injection strategy using the high-speed natural flame luminosity (NFL) imaging technique, and the flame temperature and soot volume fraction were extracted using the two-color method. NG was added into the ethanol/air mixture as an energy surplus and NG energy rate (NER) was set at 0%, 50%, 60% and 70%. The diesel pilot injection time (PIT) was set at −24°CA, −18°CA and −12°CA ATDC. Results indicate that adding surplus NG results in a delay in the ignition, growth in the combustion duration and pressure rise rate, an increase of in-cylinder pressure and indicated mean effective pressure (IMEP). Meanwhile, the addition of NG has significant effect on the flame development and soot formation characteristics but their sensitivities to the NER are PIT dependent. At −12°CA ATDC case, the increase in NER results in a substantial decrease in spatially integrated natural luminosity (SINL), flame area (FA) and total soot KL factor (TKL) peaks and a backward shift in the curves of SINL, FA and TKL. But at advanced PIT cases, the SINL, FA and TKL curves obviously grow up as the NER increases. Besides, advancing the PIT results in a longer ignition delay, shorter combustion duration and lower IMEP. Moreover, with advancing the PIT, the TKL peak and time integrated total soot KL factor (IKL) show the tendency of decrease for diesel-ethanol dual fuel. However, for diesel-ethanol-NG ternary fuel, the TKL peak and IKL first increase and then decrease.</p>