The photocatalytic oxidation of methyl orange under weak illumination conditions was carried out in two different types of fluidized bed reactors. TiO2 powder was employed as the photocatalysts and a 15 W low pressure mercury lamp was used as the light source and the reactor volume was 2.5 l. The parametric study of photocatalytic oxidation of methyl orange in two different types of fluidized beds was investigated; effect of catalyst loadings, pH, air flow rate, initial concentration and oxygen concentration on the oxidation reaction rate. The experimental results were analyzed in conjunction with the characteristics of fluidized bed and the reactor geometry effect on the reaction rate was analyzed.