The grain boundary peak in molybdenum was measured by a torsion pendenlum, when the frequency of vibration is 1 cps, the peak occurs around 1020℃. This G. B. peak is reduced by oxygen or carbon additions, but an "alloy grain boundary peak" appears at the lower temperatures on the same damping curve. Using frequency of 1 cps, "Oxygen peak" occurs around 890℃; and "carbon peak" occurs around 925℃. Since no peak is evident on the damping vs temperature curve of molybdenum single crystals, it may be concluded that these peaks are all associated with the stress relaxation across grain boundaries. If some carbon is also present in the sample containing oxygen, the other two "alloy grain boundary peaks" appear around 960℃ and 985℃. The activation energys of all the peaks were determined. The higher is the peak temperature, the larger is the value of activation energy, i.e. the activation energy of "oxygen peak" is the smallest one (80 kcal/mol); that of "carbon peak" is somewhat larger (98 kcal/mol); that of "pure grain boundary peak" is the largest (119 kcal/mol); and the activation energies of the other two "interaction peaks" are somewhat less than that of "pure grain boundary peak".Fractographic work was made after the damping measurements had been taken. Many black oxide spots were observed on the intergranular facets of the specimen, by which the "oxygen peak" had been measured; the feathery carbides were observed on the intergranular facets of the same specimen as we had measured the "carbon peak". When both oxygen and carbon are present in the specimen, which gives the "interaction peaks" around 960℃ and 985℃, many white precipitations embeded by black rings were observed on intergranular fracture facets.Based on the results presented above, the mechanism of the embrittlement of molybdenum by a small amount of interstitial solute elements was discussed, especially the fact that carbon counteracts the severe embrittling effect of oxygen was also explained.