The aim of this research is to study the effect of water absorption on the fatigue property. This is to be done for materials that made from polyester resin reinforce by carbon fibers.This material has been prepared in the laboratory as sheets with two different percentages of fiber carbon contents , These percentages are 5% and 8% . Then a specimens have been prepared in a shape and size that convenient to suit the fatigue test machine . These specimens has been weighted before and after immersed in water for (15) days. The weight of the specimens have been checked every (24) hours in order to know the water absorption at room temperature (32) c0 .Then a fatigue tests have been carried out for two types of specimens , These specimens are dry and wet specimens .