Abstract: The paper contains experimental results of propagation rate distribution of a toe crack along its front under fatigue load.Experiments were performed on aluminum alloy(LY-12R) specimens simulating geometrical shape of a T-joint with toe crack.To obtain the crack propagation rate,markings were induced on fracture surface with changes in amplitude at specified numbers of fatigue load cycles.After complete fracture of the specimen,propagation rates were calculated by measuring the distance between markings on the fracture surface.It is shown that(1) owing to the stress concentration at the toe,the propaga-tion rate of a toe crack in the direction of plate width is always higher than that of a crack of the same size in a flat plate,irrespective of the crack depth;(2) additional moment,caused by angular welding distortion,increases the propagation rate of toe crack in the direction of plate width;(3) the stress intensity factor of a toe crack at the front near the surface is always higher than that of a crack of the same size in a flat plate under the same loading conditions;(4) compressive residual stress at the toe is effective in reducing the propagation rate of toe crack;(5) the length of a crack originated from a toe crack when its vertex reaches the back surface of the plate is much greater than the final length of a part-through crack of the seme size,originated from a flate plate,when the plate is penetrated through under the same loading conditions.This should be taken into full acconnt in designing pressure vessels on the pinciple of "leak before break".