It is well-known that blue brittleness and temper brittleness of steel are related to diffusion and precipitation in steel. An attempt was made in the present research to investigate this problem by means of internal friction measurements. Experiments show that the diffusion of carbon in α-iron is not affected by carbon concentration, alloying elements and the stage of precipitation in iron. The diffusion of nitrogen in α-iron is, however, hindered by the addition of alloying elements, and is enhanced during the initial stage of precipitation.The internal friction peak (around 250℃ for a frequency of vibration of 1 cycle per second) associated with the precipitation of carbon or nitrogen in a cold worked a-iron specimen previously observed was further studied. It is found that in specimens of sufficiently high carbon or nitrogen content, the internal stress resulting from the quenching of the specimens from high temperatures also gives rise to this internal friction peak. Further experiments show; (1) this internal friction peak is associated with the phenomenon of strain aging or quench aging, and it occurs under conditions corresponding to the appearance of blue brittleness in steel; (2) the carbon or nitrogen associated with this internal friction peak is in a dispersed atomic form. This correlation suggests that the blue brittleness of steel is a process of strain aging or quench aging and it is directly associated with the separation of carbon or nitrogen atoms from solid solution into stress regions or defects in steel.A tentative suggestion concerning the mechanism of temper brittleness in steel has been made that temper brittleness is associated with the process of precipitation of nitrides instead of carbides in steel.