In order to investigate the effect of grain size on Portevin-Le Chatelier yielding, tensile tests were performed on Al-0.55 and 1 at%Mg alloys having various grain sizes.It was found that the value of ƒÃ0 was decreased and the stress at the onset of P-L effect was increased with the decrease of grain size. The above fact is explained by the effect of grain size on the average velocity of moving dislocations. (Received Aug. 20, 1970) Faculty of Engineering, Doshisha University (Kyoto)