Diesel-NG fuel blends are increasingly being used in Reactivity Controlled Compression Ignition (RCCI) internal combustion engines due to high Brake Thermal Efficiency (BTE), low NOx and PM emissions. It also has few disadvantages such as high unburned Hydro Carbon (HC) and Carbon Monoxide (CO) emissions and relatively low Exhaust Gas Temperature (EGT). This study determines the optimum combustion mode between RCCI and Conventional Diesel Combustion (CDC) at different loads while meeting the Environmental Protection Agency (EPA) emission regulation. A Cost Function (CF) including Brake Specific Fuel Consumption (BSFC) and Brake Specific Urea Consumption (BSUC) is considered and minimized in this study. The optimization of input variables is done between 3 and 12 bar Indicated Mean Effective Pressure (IMEP) engine load. The study aims to calibrate the dual fuel diesel/NG RCCI engine to meet Tier 3 Bin 20 EPA standard, with or without after treatment system, while minimizing the cost of operation. New parametric empirical models are developed and validated using experimental data from a light duty 1.9L inline 4 cylinder Compression Ignition (CI) engine as a function of independent input variables. All the experiments were conducted at Advanced Power System (APS) facility at Michigan Technological University. These models predict HC, CO, PM and NOx emissions, EGT and BSFC. These models are then used to predict new operating points to increase the population in the optimization process. The computed EGT is used to estimate the Selective Catalyst Reduction (SCR) and Diesel Oxidation Catalyst (DOC) efficiencies to assess the emission data with different input variables by considering the after-treatment system to see if they meet the tailpipe emission regulation. The optimization results recommend using Diesel/NG RCCI at 7 to 12 bar IMEP operating conditions and use CDC for below 7 bar IMEP operating condition.