The stress distribution and the amount of crack-open-up are calculated on an extending crack : with a certain constant velocity under uniform tensile stress at infinity, and the effect of crack velocity on the stress distribution and strain energy release rate is studied. The conclusions are as follows : (1) The stress distribution on the crack path is not disturbed beyound the point where the longitudinal stress wave reaches at that moment, and in the disturbed region stress becomes partially lower than the uniform stress at infinity. The minimum stress appears at the point where Rayleigh wave reaches. (2) As in the static problem, the released elastic strain energy is proportional to the square of the crack length and the strain energy release rate is proportional to the crack length. The effect of crack velocity to them is little while the velocity is not so high (α < 0.2), but when the crack propagates at the higher velocity they increases rapidly, and at the velocity of Rayleigh wave they becomes indefinite.