The irradiation effect in poly(methyl methacrylate) changed from main-chain scission to crosslinking, depending on the stopping powers of the Au and Xe ions irradiated with high stopping powers. The latent ion tracks, including an end of the ion range in the polymer, were observed clearly by field emission and atomic force microscopies. Additionally, it was also observed that a crosslinked structure (formed by nuclear stopping) existed across an end of the ion range calculated by a Transport of Ion in Matters code, and it was different from that formed by electronic stopping. The nuclear stopping of the heavy ions can play an important role around the end of the ion range in the polymer. (C) 2001 John Wiley & Sons, Inc.