© 2020 The Japan Institute of Metals and Materials. Synthesis of colloidal indium(III) oxide (In2O3) nanocrystals (NCs) by a simple non-injection method, using indium(III) oleate as an indium and oxygen source, was studied under conditions of various precursor concentrations, reaction temperatures and times. In the case of using a solution with a low precursor concentration, spherical In2O3 NCs that exhibited high crystallinity were successfully obtained. Their size was easily controllable in the range of 4 to 6nm in diameter by changing the reaction temperature and time. Because the growth of the spherical NCs was developed by the precipitation of new In2O3 components from solution and not by Ostwald ripening, the obtained NCs exhibited a narrow size distribution and were almost monodispersed. In contrast to the case of using a solution with a low precursor concentration, rod- and triple-fork-shaped NCs were obtained in the case of using a solution with a high precursor concentration. Formation of the rod- and triple-forkshaped NCs was attributed to the high growth rate after the nucleation because of the high initial precursor concentration.