Abstract: In laser proton acceleration experiments, identifying different ions in the ion products is a basic and important issue. In this paper, we accomplished a calibration experiment, in which the CR39 detector is irradiated by monoenergetic proton and carbon ion beams. With standard etching conditions we developed, their track diameter vs energy response curves were obtained. These curves were further applied to determining the energy of ions and distinguishing protons from carbon ions in our laser proton acceleration experiment. In this experiment, the diameter and relative gray of ion tracks were measured, proton tracks were picked out and proton spectra were obtained finally.