Internal friction measurements were made on commercial iron specimens containing various amounts of cerium and lanthanum (combined content: 0, 0.011, 0.026. 0.037, 0.075, 0.124%). For specimens thoroughly treated with wet-hydrogen to remove carbon and nitrogen impurities, the activation energy associated with the grain boundary internal friction peak was found to vary with the content of cerium and lanthanum. A maximum appears around the content of 0.03%. The maximum activation energy is 11.3×104 calories per mole and is about twice the value for iron free from cerium and lanthanum (6.4×104 calories per mole). For specimens loaded with carbon, the grain boundary internal friction peak was found to appear at a temperature 30℃ higher than that of the specimen free from cerium and lanthanum. The activation energy associated with the grain boundary peak was found to be 7.6×104 calories per mole (experimental error is within ±2000 calories per mole) and is approximately independent of the cerium and lanthanum content.Measurements on the Snoek peak associated with carbon in iron showed that the height of the peak decreases with an increase of the content of cerium and lanthanum, whereas the position of the peak remains unaltered as compared with that of iron specimen free from cerium and lanthanum.The cold-work internal friction peak of the original iron specimen containing carbon and nitrogen was found to appear around 230℃ (with the specimen cold-worked to 88% reduction in area), and this peak was considerably lowered with an increase of the cerium and lanthanum content. After the specimens were fully treated with wet-hydrogen and then loaded with nitrogen, the cold-work peak (with the specimen cold-worked to 88% reduction in area) was found to appear around 190℃. This peak was also considerably lowered with an increase of the cerium and lanthanum content.Preliminary discussions were made on the possible origins of the effect of the addition of rare earth elements on the three internal friction peaks (grain-boundary peak, Snoek peak, cold-work peak) of iron.