A mild crystallization process was proposed to prepare chromium trioxide (CrO3) by reaction between K2Cr2O7 and HNO3 aqueous solutions. Through ICP, XRPD, SEM, and EDS analysis, key factors and mechanisms that influenced the preparation of CrO3 were studied. Large spherical particles of CrO3 (d(50)300 mu m) with high purity (CrO(3)0.99g/g, K2mg/g) were prepared when the initial concentration of K2Cr2O7 was kept at 80g/100mL HNO3, the acid feeding rate and the cooling rate were set at 1 mLmin-1 and 0.1 degrees Cmin-1, respectively, and the direct recovery of Cr6+ was up to more than 95%. Kinetic analysis indicated that low nucleation rate and high growth rate would favour the increase of CrO3 particle size in this process. As a high-valued byproduct, Cr(VI)-free KNO3 was further prepared with the designed crystallization steps. Additionally, green characteristics of the process were also discussed.