Test restuls showed that under static loading crack growth rate decreases with increase of crack extension until the crack is arrested while under cyclic loading the crack growth with a constant velocity without being arrested even in the range of K values where no growth is observed under static loading. Crack length depen-dence of "K-V relationship" (K:stress intensity factor V: crack velocity) and crack velocity dependence of "K-$Delta$a relationship " ($Delta$a: crack extension) obtained from the crack growth data concerning the relation between crack velocity and crack extension under static loading and cyclic loadingg and cyclic loading