Abstract: To investigate the physicochemical properties and stability variations of micrometer-sized aluminum hydride (AlH₃) with different particle sizes, three AlH₃ samples with D₅₀ particle sizes of 13.1, 21.4, and 84.7 μm were characterized. Elemental analysis and X-ray diffraction (XRD) were adopted to characterize variations in crystal structure and chemical purity. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were utilized to analyze morphological features and surface elemental composition. Isothermal and non-isothermal thermal analyses were performed to evaluate discrepancies in thermodynamic stability. The results reveal that all three samples are α-AlH₃ with chemical purity higher than 95%. AlH₃ specimens of varying particle sizes possess analogous surface elemental compositions, whereas prominent distinctions exist in the crystalline surface morphology. The fine-particle AlH₃ (D₅₀=13.1 μm) contains fewer surface defects and displays a microstructure nearly identical to the ideal crystalline structure of α-AlH₃. As the particle size of AlH₃ increases, the number of crystalline surface defects rises, and severe particle agglomeration is observed for the coarse AlH₃ sample with a D₅₀ of 84.7 μm. The apparent activation energies for non-isothermal thermal decomposition of the three graded AlH₃ samples are all approximately 95 kJ/mol, presenting no statistically significant difference. Nevertheless, remarkable disparities are identified in the isothermal hydrogen desorption rates among the three samples. After isothermal aging at 50 °C for 10 days, the hydrogen desorption ratios of AlH₃ with D₅₀ values of 13.1, 21.4, and 84.7 μm reach 0.054%, 0.104%, and 0.136%, respectively.