CNG-diesel dual fuel combustion mode has been regarded as a practical operation strategy because it not only can remain high thermal efficiency but also make full use of an alternative fuel, natural gas. However, it is suffering from misfire and high HC emissions under cold start and low load conditions. As known, hydrogen has high flammability. Thus, a certain proportion of hydrogen can be added in the natural gas (named HCNG) to improve combustion performance. In this work, the effect of hydrogen volume ratio on combustion characteristics was investigated on an optically accessible single-cylinder CNG-diesel engine using a Phantom v7.3 color camera. HCNG was compressed into the tank under different hydrogen volume ratios varied from 0% to 30%, while the energy substitution rate of` HCNG remained at 70%. The results show that with the increase of hydrogen volume ratios, the peak of in-cylinder pressure and heat release increase significantly, and the crank angles corresponding to the maximum pressure, maximum heat release rate, and cumulative heat release rates of 5%, 20% and 50% advance. With the increase of hydrogen volume ratios, the ignition delay, from main injection timing to initial flame timing, decreases while the number of yellow ignition spots and the yellow ignition area increase. The HCNG has two ways to combustion, which are flame propagation and compression ignition. Based on the flame images, the combustion process can be divided into five phases: (1) ignition delay phase, (2) diesel premixed combustion phase, (3) diesel mixing controlled combustion phase, (4) HCNG premixed combustion phase, (5) remaining diesel mixing controlled combustion phase. Hydrogen has more notable effect on the early combustion than the late phase.