To meet the target of the CO2 regulations, it is mandatory to replace high-carbon fossil fuels with low-carbon fuels. Diesel/Natural Gas (NG) reactivity-controlled compression ignition (RCCI) can reduce CO2 emission, which stratifies two types of fuels with different reactivity. And also, RCCI produces less NOx and particulate matter emissions by reducing the in-cylinder temperature. However, RCCI must still be enhanced in terms of the thermal and combustion efficiencies at low and medium loads. In this work, a modified piston geometry was applied to improve the RCCI combustion. The piston geometry was designed to minimize heat loss and reduce flame quenching in an RCCI engine. Experiments were conducted using a single-cylinder engine with a displacement volume of 1,000 cc. Diesel was directly injected into the cylinder, and NG was fed through the intake port. Two different engine loads were selected to represent the low and medium loads for the operation of the engine, i.e., 4.3 bar indicated mean effective pressure (IMEP) and 7.5 bar IMEP, respectively. According to the experimental results, the combustion efficiency and thermal efficiency of the modified piston were improved by up to 0.8% and 1.0%, respectively. Energy balance analysis showed that the heat transfer loss decreased by 1.1% owing to the minimal surface area of the piston. Therefore, the thermal efficiency improved. In addition, the combustion efficiency was improved because a large squish height and a small squish distance enabled the flame to reach the crevice area, which had poor conditions for flame maintenance. Heat-loss management using an modified piston geometry can contribute to the reduction in CO2 emissions in RCCI engines.