A dual fuel operation with different reactivity fuels has the possibility of optimizing performance and emissions in premixed charge compression ignition engines by controlling the spatial concentration and distribution of both fuels.In the present study, n-heptane and i-octane were independently injected through two different injectors. In-cylinder pressure analysis and emissions measurement were performed in a compression ignition engine. Injection timings, fuel quantity ratio between the injections were changed for the two cases, in which one fuel was injected using a port fuel injection system while the other was directly injected into the cylinder, in order to drastically vary mixture distributions and ignition timings. In addition, an optical diagnostic was performed in a rapid compression and expansion machine to develop an understanding of the ignition processes of the two mixtures.The experimental results show that not only the mixedness of the more reactive fuel n-heptane but also that of the less reactive fuel i-octane have significant impacts on the ignition timings, burn rates and emissions. As a first step to establish an optimum method of controlling burn rate, engine performance and emissions formation processes, the present study developed an understanding of the effects of the mixture distributions and ignition timings of two fuels on emissions and thermal efficiency.