Bismuth molybdate single-crystallites were synthesized by a simple hydrothermal method at different pH values. The as-fabricated samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM); it has been found that the pH values of the precursors can significantly influence the structures of the products. Under the Bi/Mo ratio of 2:1 conditions (a nominal composition of Aurivillius structure), low pH leads to the formation of gamma-Bi2MoO6 with Aurivillius type structure, while neutral and higher pH values lead to the formation of gamma Bi2MoO6/Bi2O3 composite, and Bi2O3, respectively. The photocatalytic activities of the samples with different structures were measured for the degradation of methylene blue (MB) under simulated sunlight irradiation. Among the as-fabricated bismuth molybdate samples, the gamma-Bi2MoO6 exhibited superior photocatalytic efficiency to the gamma-Bi2MoO6/Bi2O3 composite and the Bi2O3, mainly due to its unique layered Aurivillius structure, smaller band gap structures and smaller particle sizes.