Nanosized SAPO-34 has been successfully synthesized by a facile and low-cost method involving an azeotrope evaporation and steam-assisted crystallization route without either expensive poregens or homemade starting materials. The as-synthesized nanosized SAPO-34 shows ultrasmall grain sizes and an impressively high total pore volume, as confirmed by X-ray diffraction, nitrogen physisorption, scanning electron microscopy, and transmission electron microscopy techniques. The formation process and the pivotal role of supersaturation in the synthesis of nanosized SAPO-34 have been revealed by a series of control experiments, on the basis of which a nonclassic oriented attachment mechanism under high supersaturation ratio conditions is proposed. When used as the catalyst for the conversion of chloromethane to olefins, the nanosized SAPO-34 exhibits acidity comparable to that of the conventional SAPO-34 but double the catalytic lifetime and a slightly lower selectivity for light olefins.