The paper describes the problem of crack propagation. The object of consideration are the common gearbox gears. From the measurements it is obvious that the crack can appear at the very different locations in tooth root and then propagate in their own direction, Pehan [1]. Theoretically it can happened in very narrow field on the tooth root surface where the principal stresses are the highest. Few reasons are discussed that can influence on the start position of crack initiation: the loading distribution over the tooth flank, the undercutting of the tooth and the tracks that remains from the cutting tool. It is confirmed that all counted phenomena can cause wide scatter of crack propagation in tooth root, Pehan [2].