For the $eta$=1 specimen with constant thickness, Crack growth rate is smoothly increasing in the a-N curve. On the other hand for $eta$=2 specimen with various thickness, the inflection point is observed in crack growth rate near the thickness interface. da/dN before the inflection point is increased, and da/dN after the point is decreased, compared to the $eta$=1 specimen. da/dN near the thickness interface is approached zero. The descending point was observed earlier as $eta$ increased. Considering the relation between da/dN and λ, the crack propagation rates for the case of $eta$ =1 incrased almost linearly, however, the crack propagation rates for $eta$=2,3 decreased more rapidly near the thickness interface. Additionally, the decreased point in da/dN for $eta$=3 is farther from the thickness interface than the case for $eta$ =2.