Carbon fiber reinforced poly(urethane-isocyanurate)-nanosilica composites CF-(PUI-NS) were manufactured by means of the vacuum-assisted resin transfer moulding technique (VARTM) at very low NS concentrations (0-4 wt%). The high strain to failure of the PUI matrix (>7%) affected tensile tests by CF reorientation. Both the tensile strength and strain to failure were highly dependent on its kinematics. CF(PUI-NS) caused an increase of the static toughness with a maximum improvement of tensile strain to failure and modulus of +28.8% and +39% at 1 wt% and 2 wt% of NS, respectively. The interlaminar shear strength (G(IC)) of the composites showed both a deterioration of -12.9% and an improvement of +9.9% for NS concentrations of 1 wt% and 4 wt%, respectively. Regardless of the G(IC) value, all of the composites prepared with NS presented secondary maxima of the force versus displacement plots, indicating a substantial arrest of the crack propagation velocity after delamination started. Fractographic analysis revealed several features, such as fiber pull-out, bridging as well as river patterns whereas the composites prepared with NS behaved in a more ductile fashion due to the presence of river patterns and a reduced fiber pull-out. POLYM. ENG. SCI., 58:1241-1250, 2018. (c) 2017 Society of Plastics Engineers