Effect of in situ formation of PA66 fibrils and modification with graphite nanoplatelets (GNP) on fracture behavior of the high-density polyethylene (HDPE)-matrix microfibrillar composites (MFC) has been evaluated using tensile impact strength (TIS) and J-integral methods. According to J-integral, the main mechanism of failure is unstable crack growth; dissimilar layout of both methods causes different contribution of reinforcement with GNP and PA66 fibrils to fracture process evaluated. More marked orientation and parallel loading lead to increase in impact energy by both GNP and fibrils in TIS; on the other hand, much lower contribution of fibres and even negative effect of GNP pre-blended in HDPE was found for J-integral as a consequence of load acting in perpendicular direction. The fact that lower addition of GNP pre-blended in PA66 does not deteriorate impact behavior indicates that such modification of MFC is an efficient way to get materials with enhanced, well-balanced parameters. POLYM. ENG. SCI., 59:382-388, 2019. (c) 2018 Society of Plastics Engineers