The solid acid catalysts SO42-/ZrO2 were prepared by impregnation technique at different calcination temperatures. The surface characterizations were carried out by using scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), temperature programed desorption of NH3 (NH3-TPD), and N-2-BET. The SEM results showed that the size of the SO42-/ZrO2 was not uniform and varied from about 1 to 20 mu m. The characteristic peaks in FTIR spectra were essentially the same within the calcination temperature range of 400-700 degrees C. The XRD results indicated that the transition temperature from amorphous to tetragonal phase was up to 500 degrees C. The strong acid and superacid sites of the samples could be observed by the NH3-TPD results. The largest BET surface area was 140 m(2)/g, when the calcination temperature was at 500 degrees C, and all the pore size distributions belong to mesoporous range. The solid acid SO42-/ZrO2 was used for the epoxidation of castor oil. When the calcination temperature of SO42-/ZrO2 was 600 degrees C, reaction temperature 45 degrees C, and reaction time 8 h, the reaction effect was better with an iodine value of 33.0 +/- 1.6 g/100 g and an epoxy value of 2.45 +/- 0.11 mol/100 g.