Abstract: Crystallization behaviors of electroless Ni-P deposits with 12％(w) P were studied by differential thermal analysis (DTA), thermogravimetry (TG) and in situ X-ray diffraction (XRD). The Ni-P alloys were prepared from a bath containing NiSO4 28 g•L－1, NaH2PO2•3H2O 32 g•L－1, C6H8O7•H2O 16 g•L－1,EDTA[CH2N(CH2COOH)2]2 7 g•L－1, C4H4O6 6 g•L－1, NaAc 18 g•L－1, C3H6O3 6 g•L－1, KIO3 0.001 g•L－1, with pH 4.5～5.0， at 90 ℃. The DTA curves of the alloys indicate that there were two exothermic processes at 350 and 420 ℃, respectively, which could be related to the processes of phases change of the alloys as the mass change is less than ±1％ as revealed by the TG curves. The results of in situ XRD show that the structure of the alloy is amorphous when the temperature bellows 300 ℃. The XRD pattern of metastable Ni5P2 and Ni12P phases initially appeared at the temperature over 320 ℃, and disappeared completely at 400 ℃. The XRD peaks belonging to Ni3P and Ni stable phases emerged at 360 ℃. The metastable Ni5P2 and Ni12P5 remained during the testing period of 2 h at 325 ℃, and lasted just for 40 min at 350 ℃ before transferring into the stable phases of Ni3P and Ni. According to the results, the exothermic peak of the DTA curves for the deposits at 350 ℃ should result from the two processes, i.e., the amorphous Ni-P alloys crystallized into metastable Ni5P2 and Ni12P5 phases, and then the metastable phases partially turned into the stable phases of Ni3P and Ni.