Abstract Reactivity Controlled Compression Ignition (RCCI) engines hold promise for decreasing NOx and particulate emissions. RCCI engines use direct injection (DI) to introduce a high reactivity fuel into the cylinder while a lower reactivity fuel is port fuel injected (PFI). A large reactivity difference between high reactive (diesel) and low reactive (natural gas) fuels provides a strong control variable for phasing and shaping combustion heat release in RCCI engines. Two diesel fuels, a High Reactive Diesel (HRD) fuel with a cetane number (CN) of 85 and a U.S. Ultra-Low Sulfur Diesel (ULSD) fuel with a cetane number of 49 were selected for this study. The effects of these two diesel fuels with methane as the low reactivity premixed fuel were compared in RCCI combustion mode and in Conventional Diesel Combustion (CDC) mode. The hypothesis is that using a HRD fuel in RCCI applications, with a double injection strategy, increases the reactivity of the mixture in the squish region, promoting combustion and consequently reducing unburned hydrocarbons and carbon monoxide. These emissions are generally difficult to control in RCCI combustion at high Blend Ratio (BR). In addition, a larger reactivity difference between the two fuels in RCCI applications, extends the combustion duration and reduces the Maximum Pressure Rise Rate (MPRR) and in-cylinder peak pressure. The reduction in MPRR in RCCI combustion mode makes it possible to operate the engine at higher engine loads without exceeding the MPRR mechanical constraint. The experiments were performed on a 1.9L inline 4 cylinder turbocharged compression ignition (CI) engine modified for dual fuel operation at an engine speed of 1500 RPM and 8 bar IMEP. A full factorial Design of Experiment (DOE) test program and analysis was conducted with four input variables including the diesel fuel reactivity, BR, Exhaust Gas Recirculation (EGR) and Direct Injection (DI) strategy and at two levels of interest. This analysis was used to quantify the impact of the independent input variables on engine out emissions, performance, and MPRR mechanical constraint. Following the DOE testing and analysis the optimum injection strategy to maximize the Brake Thermal Efficiency (BTE) was found for both fuels by sweeping the main Start of Injection timing (SOImain). The results of the DOE analysis showed that the cetane number is the most significant factor that effects HC and CO emissions. The experimental results showed that HRD fuel provides a 2% improvement in brake-thermal efficiency, a 14 g/kW.hr reduction in HC emissions, and 0.5% lower Coefficient of Variation (COV) of Indicated Mean Effective Pressure (IMEP) compared to the baseline diesel-NG RCCI combustion.