NO2-sensing properties of porous In2O3 (pr-In2O3) powders prepared by ultrasonic-spray pyrolysis employing polymethylmethacrylate (PMMA) microspheres as a template has been investigated in this study. The PMMA microspheres were synthesized in water by ultrasonic-assisted emulsion polymerization employing methyl methacrylate monomer, sodium lauryl sulfate as a surfactant and ammonium persulfate as an initiator. The PMMA microspheres synthesized was quite uniform and the particle size was ca. 60.2 nm (measured by dynamic light scattering). The microstructure of pr-In2O3 powders prepared was largely dependent on the kind of In2O3 sources. The pr-In2O3 which was prepared from In(NO3)3 as an In2O3 source (pr-In2O3(N)) consisted of submicron-sized spherical particles with well-developed spherical mesopores (several tens of nanometers in pore diameter) and each oxide wall among pores was constructed with meso-sized In2O3 particles connected continuously. On the other hand, the pr-In2O3 which was prepared from InCl3 as an In2O3 source (pr-In2O3(Cl)) was composed of a large number of dispersed meso-sized particles and a few submicron-sized dense spherical particles.In contrast, the morphology of conventional In2O3 powder (c-In2O3) prepared by ultrasonic-spray pyrolysis of PMMA-freeIn(NO3)3 aqueous solution as a reference was relatively dense and roughly-spherical with a diameter of ca. 100~700 nm. The responses to 1.0 and 10 ppm NO2 of pr-In2O3 sensors in air were much larger than those of a c-In2O3(N) sensor in the temperature range of less than 250°C and 300°C, respectively. In addition, the response and recovery speeds of both the pr-In2O3 sensors were much faster than those of the c-In2O3(N) sensor, because of the well-developed porous structure of the pr-In2O3 sensors.