Nano-sized MgO particles with well-crystallized structures were prepared by spray pyrolysis from spray solutions with citric acid. The optimum concentration of citric acid to prepare micrometre-sized MgO precursor particles in the spray pyrolysis was 0.4 M. The addition of citric acid to the spray solution affected the thermal properties and morphologies of the precursor particles. The micrometre-sized precursor particles with hollow and porous structures obtained by spray pyrolysis turned to nano-sized MgO particles after post-treatment. The post-treated MgO particles had aggregation-free nanometre sizes after simple milling by hand using an agate mortar. The mean sizes of the MgO particles measured from high magnification TEM photographs at post-treatment temperatures of 500 and 900oC were 13 and 28 nm. The addition of citric acid to the spray solutions decreased the mean sizes of the primary particles at the same post-treatment temperatures. The mean crystallite sizes of the post-treated particles obtained from the spray solution with citric acid at post-treatment temperatures of 500 and 900oC were 9 and 22 nm.

Keywords: spray pyrolysis, nano particle, magnesium oxide.