The present experimental study explores the effects of compression ratio and piston design in a heavy-duty diesel engine operated with Reactivity Controlled Compression Ignition (RCCI) combustion. In previous studies, RCCI combustion with in-cylinder fuel blending using port-fuel-injection of a low reactivity fuel and optimized direct-injections of higher reactivity fuels was demonstrated to permit near-zero levels of NOX and PM emissions in-cylinder, while simultaneously realizing high thermal efficiencies. The present study consists of RCCI experiments at loads from 4 to 17 bar indicated mean effective pressure at engine speeds of 1,300 and 1,700 [rev/min]. The experiments used a modified piston to examine the effect of piston crevice volume, squish geometry, and compression ratio on performance and efficiency. Results from a bathtub-style piston with reduced crevice height, short squish length, and a 14.88:1 compression ratio were compared with results from an open-crater-style piston with both 16.1:1 and 11.6 compression ratios, and a piston with a PCCI-style narrow bowl with 15.5:1 compression ratio. Of all the tested pistons, the bathtub-style piston was found to offer the highest brake efficiency and to enable low emissions and low pressure rise rates with practical intake and exhaust gas recirculation (EGR) temperatures (e.g., 70°C and 120°C, respectively). The experiments also demonstrated that engine-out unburned hydrocarbon emissions (HC) were highly correlated with crevice volume, but that optimizing the squish geometry could significantly reduce engine-out HC emissions, even with a large crevice volume. The study also found that the reduced compression ratio, bathtub-style piston decreased peak cylinder pressure, which is expected to increase brake efficiency by lowering friction losses. Engine operation with the stock and bathtub pistons was demonstrated to meet EPA HD 2010 NOX and PM mandates in-cylinder (no after-treatment required) and to provide a 5-15% improvement in the estimated brake efficiency compared to conventional diesel operation with 16.1:1 compression ratio and 50% overall turbocharger efficiency, which requires after-treatment for soot and NOX reduction.