Abstract: OBJECTIVE To study the structure characterization and accelerated stability of tacrolimus monohydrate (tacro-A and tacro-B) with different crystal habits. METHODS The crystal structure was characterized by X-ray single crystal diffraction and X-ray powder diffraction combined with BFDH model theory. The micro crystal morphology of the two samples were obtained by scanning electron microscopy. The stability of the two crystals were investigated by 6-month accelerated stability test. RESULTS The column shape tacro-B was better than the sheet shape tacro-A in the accelerated stability test, and there was no new impurity in tacro-B after 6 months of accelerated stability test. CONCLUSION It is an important way to change the drug properties such as solubility and stability by adjusting its external structure (such as crystal habit) while keeping the internal structure (i.e. crystal form) unchanged.