New applications of elastomers, such as flexible electronics and soft robotics, have brought great attention to tear resistance since elastomers are prone to shear failure. Most elastomers contain chemical cross-links and entanglements. The effects of both on their mechanical properties have been intensively studied, while how they cope with tearing remains elusive. Here, in polybutadiene elastomers, we find that the energy release rate of tearing (Gtearing), often employed as a measure of tear resistance, is influenced synergistically by chemical cross-linking and entanglements, while its threshold (G0) is only related to the chemical cross-linking. At a low tear speed, the polybutadiene elastomers with low cross-linking density have Gtearing up to 4 times higher than their G0 compared to highly cross-linked ones. Different from conventional reinforcement due to volume dissipation of a polymer network, enhancement of Gtearing significantly depends on the degree of cross-linking. The enhancement of Gtearing at low cross-linking degrees may be related to a novel mechanism, the friction-strengthening phenomenon, which was possibly caused by the pull-out of the chains at a high degree of orientation.