Depth uniformity of electrical properties has been evaluated for neutron‐transmutation‐doped (NTD), semi‐insulating GaAs irradiated with thermal neutrons of 1.5×1018cm−2by the van der Pauw method combined with iterative etching of the surface. In NTD‐GaAs wafers (thickness ∼410 &mgr;m) annealed for 30 min at 700 °C, the depth profiles of the resistivity, the carrier concentration, and the Hall mobility show constant values of 1×10−2&OHgr; cm, 2.0×1017cm−3, and 3100 cm2/V s, respectively, within an experimental error of 5%. In an annealing process, the redistribution and/or the segregation of NTD impurities is not observed. We also discuss the limitations of low‐level NTD in semi‐insulating GaAs. It is suggested that the activation of the NTD‐impurities below ∼1×1016cm−3is mainly restricted by the presence of the midgap electron trap (EL2).