The fatigue crack growth experiments of 2024-T4 aluminum alloy were carried out to study the thickness effects on the fatigue crack growth rate. Round compact specimens with two different thickness, 3.8mm and 12.5mm, were subjected to Mode I loading with four R-ratios (0.05, 0.1, 0.5 and 0.75) and loading amplitudes. An optical reading microscope with a magnification of 40 was used to measure the crack length. Stable crack growth is characterized by the standard form of the Paris law, material constants of the Paris law corresponding to each R-ratio were obtained by fitting the experimental data. The fatigue crack growth rate of specimens with a thickness of 12.5mm is apparently higher than that of specimens with a thickness of 3.8mm when R-ratio is equal to 0.1, 0.5 and 0.75. While the effect of thickness is relatively less significant for the case of . It can be concluded that the fatigue crack growth rate increases with R-ratio or thickness when one of them is identical.