Particle formation techniques using supercritical fluid are simple processes that can control particle size and morphology, although high-pressure is required. The purpose of this study was to investigate how the experimental conditions affect the extent and rate of CaCO3 conversion and the size and morphology of the precipitated CaCO3 from the carbonation tests with rapid spraying of reactants causing rapid depressurization of supercritical fluid. The relatively low temperature and pressure conditions (35 °C and 7.5MPa) resulted in low CaCO3 conversion efficiency (41.4-51.9%), high vaterite content (70-78%) of CaCO3, and smaller-sized particles. The relatively high temperature and pressure conditions (80 oC and 12.0MPa) resulted in high CaCO3 conversion efficiency (66.8-73.2%), high calcite content (50-80%) of CaCO3, and larger-sized particles. The particle size of solid products ranged between 20 and 180nm with approximately a peak of 100 nm in the particle size distribution (PSD) curve, irrespective of the test conditions; however, shorter reaction times led to smaller particles. The optimal conditions under which the extent of CaCO3 conversion and calcite content were maximum were 50 °C, 9.0MPa, and 1 h of reaction time (CaCO3 conversion: 92.9%; calcite content of CaCO3: 87%).