Lanthanum-doped sodium tantalate samples (NaTaO3:La) were prepared by a flux method using a sodium chloride flux with various parameters, such as presence or absence of the flux, solute concentration, hold temperature, and amount of lanthanum" doping. SEM images showed cubic and rectangular shapes for the samples prepared by the flux method, somewhat rounded shape for the sample prepared in the absence of the flux, and large particles for the sample without lanthanum doping. Among the parameters, the lanthanum doping and solute concentration much influenced the crystallites size of the NaTaO3:La samples. Most of these NaTaO3:La samples loaded with platinum cocatalyst exhibited the photocatalytic activity in the photocatalytic steam reforming of methane around room temperature. Among them, the highest activity was obtained by the Pt/NaTaO3:La sample prepared by the flux method with moderate solute concentration, enough high hold temperature, and moderate amount of platinum and lanthanum doping. A positive correlation was found between the crystallite size and the photocatalytic activity. When we compared the catalysts having the same crystallite size, the sample prepared by the flux method showed higher photocatalytic activity than the catalyst prepared without the flux. It is suggested that the difference in the shape of particle would be important factor for the photocatalytic activity. (C) 2015 Elsevier B.V. All rights reserved.