Background. Various chemical agents have been used as an adjuvant treatment for giant cell tumor (GCT). However, the comparative effect of these chemicals remains unclear. Methods. Multinucleated and spindle cells from cultured GCT patients, characterized by Nanog and Oct4 expression with RT-PCR, were directly administered, in vitro, with concentrations of 1%, 3%, and 5% of H2O2 and 75%, 85%, and 95% of ethanol for 10 minutes and concentrations of 0.003%, 0.005%, 0.01%, 0.03%, 0.1%, and 0.3% of H2O2 for 5 minutes and were incubated for 24 hours. Cell morphology, cell viability, and flow cytometry after various concentrations of H2O2 and ethanol exposure were assessed. Results. H2O2 in all concentrations caused loss of cell viability. The number of viable cells after H2O2 exposure was related to the concentration-dependent effect. The initial viable spindle-shaped cell, multinucleated giant cell, and round-epithelioid cell had morphological changes into fragmented nonviable cells after exposure to H2O2. Flow cytometry using Annexin V showed cell death due to necrosis, with the highest concentration amounting to 0.3%. Conclusion. Administering local chemical adjuvants of H2O2 in vitro caused loss of viable GCT cells. The number of viable cells after H2O2 exposure was related to the concentration-dependent effect, whereas reducing concentration of H2O2 may cause loss of viability and morphology of cultured GCT cells with the apoptotic mechanism.