One of the most promising alternative combustion strategies is natural gas/diesel dual-fuel combustion. It consists of preparing a premixed mixture of a gaseous fuel and air, whose ignition is triggered by the injection of a small amount of more ignitable fuel, usually diesel fuel. However, this combustion mode still suffers from low thermal efficiency and high level of unburned methane and CO emissions at low load conditions. The present paper reports the results of an experimental and numerical study on the effect of diesel injection timings (10 to 50 °BTDC) on the combustion performance and emissions of dual-fuel combustion at 25% engine load. Analysis of OH spatial distribution shows that, at very advanced diesel injection timings, the non-reactive mixture zones are much lower in OH concentration than other injection timings during the last stages of combustion, indicating a more predominant premixed combustion mode. At retarded diesel injection timings, the consumption of premixed fuel in the outer part of the charge is likely to be a significant challenge for dual-fuel combustion engine at low load conditions. However, with advancing the diesel injection timing, the OH radical becomes more uniform throughout the combustion chamber which confirms that high temperature combustion reactions can occur in the central part of the charge. NOx, unburned methane, and CO emissions are reduced while at the same time the highest indicated thermal efficiency is achieved at very advanced diesel injection timings of 46 and 50°BTDC.