The effect of specimen thickness on fatigue crack growth behavior has been carried out by compact tension specimens of thickness of 3㎜, 10㎜ and 25㎜ for maraging steel and Al 7075-T6. The closure points were determined during the test by means of a clip-gage situated at the notch mouth. Specimen thickness have no apparent influence on the fatigue crack growth rate of maraging steel, but the crack growth rate of 25㎜ thickness specimen for A1 7075-T6 is faster than that of 3 and 10㎜ specimens. The difference of crack growth rates can be successfully explained by considering the different stress state of plane strain and plain stress due to the variation of specimen thickness. Also the crack opening ratio of 25㎜ specimen is greater than those of 3 and 10㎜ specimens. When a side groove is introduced in a 10㎜ specimen, the crack growth rate is approximately similar to that of 25㎜ specimen. The effective thickness expression of B_e=B_o-(B_o-B_N)²/B_o is the most appropriate to evaluate the crack growth rate of side-grooved specimen. Fatigue crack growth rates can be well described by ΔK_(eff) of the crack closure points in regardless of all thickness and side-grooved specimens.