The heat capacity of single-crystalline samples of Sr 8 Ga 16 Ge 30 (SGG) and Ba 8 Ga 16 Ge 30 (BGG) clathrates was measured to investigate the anharmonicity of the encapsulated atoms. At low temperatures, BGG can be well described by a standard Debye model, and the C p /T 3 versus T plot can be fitted with two Einstein temperatures: h E1 = 42 K and h E2 = 74 K. On the other hand, SGG shows deviation from the Debye model. Moreover, neither the Einstein model nor the soft potential model (SPM) alone can fit the peak in the C p /T 3 versus T plot, and the peak should be fitted by employing both models.Our results indicate that the effective electron mass is enhanced by the anharmonic phonons.