To improve dissolution properties of aprepitant (APR) as a poorly water-soluble drug, Rapid Expansion of Supercritical Solution with Solid Cosolvent (RESS-SC) was applied for the first time. Effects of four different parameters including temperature (308.2-338.2 K), pressure (15-33 MPa), spraying distance (1-10 cm) and nozzle diameter (150-450 mu m) were investigated on the size and morphology of the precipitated nanoparticles. Also, equilibrium solubilities of APR in CO2 and menthol were measured at different temperatures between 308.2 and 338.2 K and pressures in the range of 12-33 MPa using a static method. To characterize unprocessed and processed APRs, FTIR, DLS, SEM, XRD, DSC and dissolution rate were employed. The analyses revealed a great reduction in the size of the nanoparticles by RESS-SC (23 +/- 1.6 nm, on average), as compared to the original particles of APR (25.6 mu m, on average). Finally, the dissolution rate was enhanced by 8.2 folds following the RESS-SC process.