In this paper, EPR spectra of eight olassaele YAG crystals at room temperature and 77K have been described. Based on data of EPR and optical spectra, three paramagnetic colours exoisting in crystals are suggested. Two EPR spectra with g-2 and g-1.98 are corresponding to 2×104 cm-1~3.4×l04cm-1 bands of optical absorption spectra. These gpeotra are originated from the defect centres of electronic type when the defect capturing one electron in YAG crystals. The other EPR spectrum with g~3.6 depends on oxygen vacacies and doped with metallic ions Mg2+ and Cr3+. The author supposed that it is a complex paramagnetic defect centre of S>1/2, consisting of a oxygen vacancy or its group and metallic ions. When YAG crystals doped with metallic ions and annealed in high temperature atmospheric, the colour centre concentration increases, which is caused by out of balance on the electron and volume in crystals.