Ultrafine MnFe2O4 powder with its crystallites less than 100 nm was prepared using a combustion process. The coprecipitates containing the stoichiometric amount of Mn2+ and Fe3+ to form MnFe2O4 were prepared by dissolving the required metallic nitrates in de-ionized water and adding NH4OH to adjust the pH of the solutions to 9. The collected dried precipitates were then heated up to predetermined temperatures and then quickly contacted with the acetone spray. Upon contacting with the heated precipitates, the acetone spray was ignited. The combustion of acetone caused the precipitates to form crystalline MnFe2O4 without chemical segregation. The crystallinity of MnFe2O4 powder so obtained depended on the ignition temperature of acetone spray. MnFe2O4 powder obtained at acetone ignition temperature of 773 K had higher crystallinity than that obtained at acetone ignition temperature of 523 K. The presence of a small amount of Mg2+ or Zn2+ in the composition of the coprecipitates promoted the mobility of constituent ions of the combusted powder and resulted in bigger MnFe2O4 crystallites at a lower acetone ignition temperature.