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The effect of the substitution of a rare-earth element for a guest element on the electronic and thermoelectric properties is investigated for clathrates with the nominal composition Ba 8¹x Eu x Cu y Si 46¹y (x = 0, 1, 2; y = 4, 5, 6). The Seebeck coefficient and the electrical conductivity of the rare-earth-substituted Ba 8¹x Eu x Cu y Si 46¹y samples are consistent with n-type conduction and metal-like behavior. The Hall carrier mobility of Ba 8¹x Eu x Cu 6 Si 40 at room temperature decreases with increasing Eu substitution. The conduction band effective mass of rare-earth-substituted Ba 8¹x Eu x Cu 6 Si 40 (x = 1, 2) is experimentally estimated to be larger than that of Ba 8 Cu 6 Si 40 . On the basis of ab initio electronic structure calculations for Ba 6 Eu 2 Cu 6 Si 40 and Ba 8 Cu 6 Si 40 , it is deduced that the conduction band edges of Ba 8 Cu 6 Si 40 are modified by the substitution of Eu for the guest atom Ba, resulting in an increase in the density-of-states effective mass, which is consistent with the experimental result.