A bisphenol A diglycidylether (DGEBA) epoxy was modified with up to 20 wt% of a core-shell rubber (CSR), and mechanical properties were investigated at different crosshead rates (from 0.5 mm/min to 500 mm/min). The yield strength and Young＇s modulus decreased almost linearly with increasing rubber content or decreasing crosshead rate. Fracture toughness, K-IC, was significantly improved by the addition of CSR to the pure epoxy. The optimum rubber content was between 15% and 20% at the lowest crosshead rate (i.e. 0.5 mm/min) and shifted to higher rubber content at higher crosshead rates. K-IC decreased slightly as the crosshead rate was increased from 0.5 mm/min and dropped significantly when the crosshead rate approached 500 mm/min. Rubber cavitation-induced local plastic deformation was identified as the principal energy absorption mechanism, and it was independent of rubber content and crosshead rate. The variation of K-IC with rubber content and crosshead rate correlated well with the length of the stress-whitened zone.