Due to the large diameter and small surface, the contaminant degradation by conventional calcium peroxide (CaO2) is slow with high dosage required. The aggregation of conventional CaO2 also makes it difficult to operate. Nanoscale CaO2 was therefore synthesized and applied to remove toluene in this study. Prepared from nanoscale Ca(OH)2 and H2O2 in the ratio of 1:7, the finely dispersed nanoscale CaO2 particles were confirmed by the scanning electron microscope to be in the range of 100-200nm in size. Compared to their non nanoscale counterparts, the synthesized nanoscale CaO2 demonstrated a superior performance in the degradation of toluene, which could be eliminated in 3d at pH 6. The oxidation products of toluene were analyzed to include benzyl alcohol, benzaldehyde and three cresol isomers. With the addition of 2-propanol, hydroxyl radicals were indicated as the main reactive oxygen species in the oxidation of toluene by nanoscale CaO2. Superoxide radicals were also investigated as the marker of nanoscale CaO2 in the solution. Our study thus provides an important insight into the application of nanoscale CaO2 in the removal of toluene contaminants, which is significant, especially for controlling the petroleum contaminations.