NiFe₂O₄ powders with large-sized, hollow, and thin wall structures were directly prepared by spray pyrolysis from a spray solution containing citric acid. These as-prepared powders converted into nanosized NiFe₂O₄ powders after post-treatment at high temperatures between 600 and 1000°C. Phase-pure NiFe₂O₄ powders were obtained at post-treatment temperatures between 700 and 1000°C. When the post-treatment temperatures were increased from 700 to 1000°C, the BET surface areas of the NiFe₂O₄ powders decreased from 23.4 to 11.8 m²/g and their mean sizes increased from 47.8 to 94.7 nm, respectively. The saturation magnetizations of the powders changed from 32 to 44 emu/g according to changes in the post-treatment temperatures. The coercivities of the powders post-treated at temperatures of 700 and 800°C were 118 and 130 Oe, respectively.