Information about correlation of material properties parameters (i.e., crystallite and particle sizes) and photocatalytic activity of tungsten trioxide (WO3) particles are still lacking. For this reason, the purpose of this study was to synthesize WO3 particles with controllable crystallite (from 18 to 50 nm) and particle sizes (from 58 to 677 nm) using a spray-pyrolysis method and to investigate correlation of crystallite/particle size and photocatalytic activity. To gain control of crystallite/particle size, synthesis temperature (120-1300(degrees)C) and initial precursor concentration (2.5-15 mmol/L) were investigated, which were then compared with the proposal of the particle formation mechanism. The results showed that both crystallite and particle sizes played an important role in photocatalytic activity. In this research, the optimum condition to produce the highest photocatalytic performance of WO3 particles was at the temperature of 1200(degrees)C (crystallite size: 25 nm), and initial concentration of 10 mmol/L (particle size: 105 nm). (c) 2013 American Institute of Chemical Engineers AIChE J, 60: 41-49, 2014