Abstract: To study the relationship between the structure and the control conditions of molybdenum oxide synthesis, the hexagonal molybdenum trioxide (h-MoO₃) microbars and the orthogonal molybdenum trioxide (α-MoO₃) nanobelts were prepared by simple hydrothermal method using ammonium paramolybdate as the molybdenum source without any structure guide agent. The phase composition and morphology of the synthesized MoO₃ were characterized by X-ray diffraction, scanning electron microscope, and the comprehensive thermal analysis. The relationship between the acidity coefficient and the MoO₃ phase transformation and morphology change was studied, and the phase transformation mechanism was discussed. The results show that MoO₃ with the different phase structures and morphologies can be obtained by controlling the acidity coefficient during the hydrothermal synthesis. h-MoO₃ is obtained when the acidity coefficient is less than 12.0, and gradually converts to α-MoO₃ in case that the acidity coefficient is more than 12.0; h-MoO₃ gradually decreases, and when the acidity coefficient is 48.0, h-MoO₃ completely disappears and α-MoO₃ is obtained. After the acidity coefficient is more than 48.0, the morphology of α-MoO₃ changes from microbar to nanobelt, which grows along the c axis with the increasing aspect ratio, thus obtaining the one-dimensional nanomaterials.