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The morphology evolution from monoclinic molybdenum trioxide (β-MoO3) to orthorhombic molybdenum trioxide (α-MoO3) and quantitative analyses of their mixtures were examined. It was found that the morphology (from spherical to elliptical shape) and color (from green to white) displayed obvious changes when β-MoO3 converted to α-MoO3 in ambient air at 773 K. The transformation from β-MoO3 to α-MoO3 resulted from a change of the internal crystalline structure. The mass percent of β-MoO3 in MoO3 mixtures showed an excellent linear relationship with the relative intensity ratio of the strongest peaks in X-ray diffraction patterns. This approach provides a simple and time-saving method to evaluate the amount of β-MoO3, which is a promising material in catalyst and electrochemical applications, in such mixtures. This finding may provide guidance for the analysis of catalytic performance of MoO3 mixtures. In addition, it was found that β-MoO3 can be easily decomposed into suboxides such as MoO2 and Mo4O11 in pure argon gas atmosphere. The possible decomposition mechanism of β-MoO3 is discussed.