Pure mode II fracture toughness of polymethyl methacrylate and its components has been studied by essential work of fracture (EWF) approach via experimental and numerical methods. EWF fracture tests with double edge notched tension (DENT) were performed on the RT-PMMA specimens at room temperature. In this investigation, the mode II fracture of polymethyl methacrylate/graft-acrylonitrile butadiene styrene (PMMA/g-ABS) blends with different weight percentage of rubber (0, 10, 15, and 20) and the thickness of samples 0.8 and 4 millimeters was investigated. The results showed that the value for the specific essential work of fracture given by including lower ligament length may be more accurate, because the ligament is completely yielded. The results also showed that for the load-displacement curves have self-similarity in shapes for the specimens with different rubber content, specimen thicknesses, and ligament lengths and the specific work of fracture (wf) increases significantly with the increasing of rubber content. The non-essential work of fracture (βwp) increases with the increasing of rubber content and the highest value belong to 20% composition in which for both thickness. The highest value of the essential work and the non-essential work of fracture belong to 20% composition in 0.8 mm specimen thickness 98.67 kJ/m2 and 99 kJ/m2, respectively. By changing the thickness of the samples the amount of the essential work of fracture showed significant changes.