In this study, influences of hybrid filler networks of carbon nanotubes (CNTs) and carbon black (CB) on fracture resistance of styrene-butadiene rubber (SBR) composites were well investigated. The spherical CB was partially substituted by fibrous CNTs in two different ways: unequal replacement (1 phr CNTs replacing 4 phr CB) and equal replacement (1 phr CNTs replacing 1 phr CB). The J-integral tests were employed to evaluate the crack initiation and propagation resistance. The strain amplification and distribution near the crack tip was measured by digital image correlation to explore the fracture resistance mechanism. Results revealed that the fracture resistance was effectively improved by unequal replacement. Meanwhile, for unequal replacement, higher content of CNTs resulted in a weaker strain amplification factor and larger amplification area near the crack tip, which contributed to the improved fracture resistance. However, opposite trends were observed for equal replacement. A synergistic effect in fracture resistance of SBR composites between CB and CNTs was realized for unequal replacement. (C) 2016 Society of Plastics Engineers.