Ultrathin InOOH nanowires with a diameter of similar to 2 nm and a length up to similar to 200 nm have been synthesized by a hydrolysis reaction in a solution system. Their transformations to c-In2O3 nanocrystals and rh-In2O3 nanowires have been investigated. A dissolution-recrystallization (in solution) or size-related decomposition-recrystallization (in air) mechanism is indicated for the former transformation; the size- and surface-determined transformation is proposed for the latter. These results will help to understand and control the phase stability and transformation concerning the nanodimension and surface restrictions, especially for ultrathin nanowires.