LaBr3∶Ce single crystal has excellent scintillation property, however, some micro-defects, such as inclusions, often appear in the crystal. The inclusions are harmful for the scintillation property of crystals. In this study, the LaBr3∶Ce crystals were grown with vertical Bridgman method. It was found that the inclusions mainly appear at the tail end of the crystal ingot. Polarization microscopy shows that there are two kinds of inclusions, one is bubble, and the other is solid inclusion. X-ray diffraction, differential thermal analysis and Raman spectroscopy were used to measure the samples with inclusion and without inclusions. However, no difference could be identified among the two types of samples. Therefore, it is conjectured that the solid inclusions are composed of substances which are similar to the matrix in composition and crystal structure. Based on these measurement results, the formation mechanism of the inclusions is explained with the constitutional supercooling theory.