The thermal expansion of nitroguanidine (NQ) crystal was investigated by means of in situ powder X-ray diffraction method and in situ FTIR spectra method. Results show that the average thermal expansion coefficients of the a-, b-, c-axis at 30-160 ℃ for NQ are 12.9×10^-6℃^-1, -10.1×10^-6℃^-1 and 145.5×10^-6℃^-1, respectively, revealing that the thermal expansion of NQ is obviously anisotropic and thermal expansion along the b -axis is negative. The anisotropic thermal expansion of NQ is caused by the anisotropic intermolecular interaction. The intermolecular hydrogen bond decreases with the increase of temperature, and the intermolecular distance increases, and the space hindrance of the molecules along the b axis is reduced, which leads to a negative expansion along the b axis.