This study involves using nickel chloride solution as a raw material to produce nano-sized nickel oxide powder with average particle size below 50 nm by the spray pyrolysis reaction. The influences of reaction parameters on the properties of the produced powder are examined. These parameters include concentration of raw material solution and atmospheric air pressure. When the nickel concentration of the raw material solution is 30 g/l, the average particle size of the powder is below 20 nm and the particle size distribution appears to be relatively regular. When the nickel concentration increases to 100 VI, particles of sizes ranging from 20 nm to 50 nm coexist. When the nickel concentration increases to 150 g/l, the average particle size reduces to about 20 nm. When the nickel concentration increases to 220 g/l, the particles of sizes ranging from 20-30 nm to 80-100 nm coexist and show a very uneven particle size distribution. As the air pressure increases from 0.1 kg/cm(2) to 3 kg/cm(2), the average particle size of the powder decreases while the particle size distribution gradually becomes more even. When the air pressure is at 0.1 kg/cm(2), the particle size distribution appears to be 20-80 nm. When the air pressure increases to 05 kg/cm(2) and 1 kg/cm(2), the particle size distribution becomes 20-50 nm and 20-40 nm respectively. When the air pressure increases to 3 kg/cm2, the average particle size becomes about 25 nm. (C) 2012 Elsevier B.V. All rights reserved.