Linear low density polyethylene/isotactic polypropylene (LLDPE/iPP) blends, with oriented microfibrils of iPP dispersed in the nearly isotropic LLDPE matrix, has been prepared via melt extrusion drawing and subsequent thermal treatment at 160 degrees C to melt LLDPE matrix. The presence of oriented microfibrils of iPP in the LLDPE/ iPP, blends not only promotes the homogenous deformation; with no drop of nominal stress around yield point, but also enhances the fracture toughness significantly. The specific Essential Work of Fracture omega(e), which is a pure crack resistance parameter per ligament area unit, is 24.7 and 33.6 N/mm for the blends with 15 and 30 wt % microfibrils of iPP, respectively. Moreover, with the deduced deformation parameters, such as true yield stress and strain hardening modulus, the relationship between deformation parameters and fracture toughness is explored. It is demonstrated that the fracture toughness can be well correlated with the ratio of true yield stress to strain hardening modulus sigma(ty)/G, and either a decrease in yield stress or an increase in strain hardening can improve fracture toughness. (c) 2007 Wiley Periodicals, Inc. J Appl Polyrn Sci 104:1291-1298,2007.