This article studies the thermostability and the crystal structure of a new anticancer drug dasatinib.  The thermostability of dasatinib was analyzed using the differential scanning calorimeter (DSC) and thermo gravimetric analyzer (TGA), and the structural characteristics of polymorphism and crystalline transformation was determined using the X-ray powder diffractometry (XRD) with in-situ high temperature accessories.  The results showed that dasatinib has at least two different crystal forms.  The form-I has one crystalline water and form-II one and half, and in a heating-up processing both of them would change their crystal structures.  After losing their crystalline water, both would change into the same crystalline form with no crystalline water.  Their melting points were almost the same: form-Ⅰ was 285.68 ℃ and form-Ⅱ was 285.50 ℃.  The results of the study method would provide a comprehensive reference for the quality evaluation of dasatinib.