The toughness of ultra high molecular weight polyethylene (UHMWPE) before and after annealing process in water was investigated by the essential work of fracture (EWF) method. Annealing in water at 80 degrees C for various aging periods of the 5 mm thickness with various ligament lengths single edge notched tension (SENT) specimens was performed in hot water. Tensile tests were performed at 2 mm/min constant deformation rate at room temperature in order to determine EWF parameters. After tensile tests, the fracture cross-sections of SENT specimens were investigated by scanning electron microscope (SEM). From the 60th day of annealing process in water, it was seen that the fracture toughness of the material decreased while water absorption in samples increased.