Polyamide 6 (PA6) and short glass fiber (SGF) reinforced PA6 composites toughened with glycidylisobutyl oligomeric silsesquioxane (G-POSS) and 1.1'-carbonyl-bis-caprolactam (CBC) were prepared by melt-extrusion compounding process and then compounds were formed to 2 mm thick double edge notched tension samples (DENT) by injection moulding process. Fracture properties of composites were analyzed by the essential work of fracture (EWF) method. Incorporation of G-POSS and CBC with 1% weight ratio to PA6 matrices individually caused a remarkable enhancement on specific essential work of fracture, w(e), value of neat PA6. It was also found that the EWF method is incapable to characterize fracture response of both 10 and 20 wt% SGF reinforced PA6 composites without G-POSS and CBC due to brittle fracture event. But the incorporation of G-POSS and CBC into the SGF reinforced PA6 matrix led to a transition on the fracture event from brittle to ductile. Accordingly, the EWF method became an applicable model for G-POSS and CBC modified, SGF reinforced PA6 composites. Increase on the tensile strength of SGF reinforced PA6 composites with 1 wt% CBC is also another outstanding enhancement in this study. (C) 2014 Elsevier Ltd. All rights reserved.