Tin oxide nanocrystals with high surface area were first synthesized in aqueous solutions at 50°C. The Brunauer–Emmett–Teller (BET) surface area successfully reached 194 m2/g. It was much higher than that of SnO2 (BET 47.2 m2/g), SnO2 (BET 25.9 m2/g), SnO2 (BET 23 m2/g), and In2O3–SnO2 (BET 3–6 m2/g). N2 adsorption characteristics revealed that they had pores of 1–3 nm, which contributed to the high surface area. Transmission electron microscopy, electron diffraction, and X‐ray diffraction indicated the morphology, crystal structure, and chemical composition of nanocrystals. Novel process allowed us to avoid sintering and deformation of the crystals, and hence realized a high surface area and unique morphology.