The bulk etch rate, VB, in CR-39 was determined by using three methods at different etching conditions. The study was carried out for alpha-particles of energies ranging from 2.35 to 4.3 MeV at three different dissolved layers of 5.4, 7.2 and 9 μm. Based on published REL-data, a very simple empirical formula was given to calculate REL for different ions of energies ranging from 0.02 to 20 MeV/nucl in CR-39, CN-85 and PC. The variation of the sensitivity V of CR-39 detector at different conditions was studied and the results were summarized in an empirical formula.