Dual-fuel combustion combining a premixed charge of compressed natural gas (CNG) and a pilot injection of diesel fuel offer the potential to reduce diesel fuel consumption and drastically reduce soot emissions. In this study, dual-fuel combustion using methane ignited with a pilot injection of No. 2 diesel fuel, was studied in a single cylinder diesel engine with optical access. Experiments were performed at a CNG substitution rate of 70% CNG (based on energy) over a wide range of equivalence ratios of the premixed charge, as well as different diesel injection strategies (single and double injection). A color high-speed camera was used in order to identify and distinguish between lean-premixed methane combustion and diffusion combustion in dual-fuel combustion. The effect of multiple diesel injections is also investigated optically as a means to enhance flame propagation towards the center of the combustion chamber. These effects are studied optically through use of temporally resolved natural luminosity imaging. Corresponding cylinder pressure and heat release rate data was also acquired and combustion phasing analysis was performed to accompany the high-speed imaging data. The images showed that double diesel injection leads to much better flame propagation towards the center of the bowl. A detailed discussion on the differences between various equivalence ratios for CNG/diesel dual-fuel combustion in terms of combustion characteristics as well as flame behavior have been presented.