The radial core size of tracks in CR-39 plastics has been determined for several heavy ions, O (8.6 MeV), Ne (18.8 MeV), Si (27 MeV), Ar (35 MeV) and Fe (80 MeV), by AFM and UV methods. In the AFM method, the core radii were assessed from the growth behavior of minute etch pits within 1 min of etching. These were derived from each intersection of the extrapolated line, fitted to each growth curve of etch pit radius. With the UV method, changes in the UV–visible spectra of CR-39 with fluence were obtained. Then the critical fluence where the track overlapping became statistically significant was determined for each ion. Based on the track overlapping model, the track core radius was calculated. Both methods were found to give results concordant with each other for these heavy ions.