The influences of loading rate and thickness on fracture behavior and mechanism of thermoset epoxy resin with polyamine hardener under mixed-mode (mode I/II) loading have been studied at low thickness and low loading rate (LTLL), as well as high thickness and high loading rate (HTHL). Under the variation of mixed-mode loading from mode I to mode II, fracture toughness of HTHL specimens were under plane-strain condition. For LTLL specimens, the fracture toughness at dominated mode I loading was under plane-stress condition, whereas those at dominated mode II loading were under plane-strain condition. The stretched zone due to the principal stress in the normal direction to the crack plane as well as shear lips due to the Poisson contraction in the thickness direction were the main characteristic of the fracture surface of LTLL specimen tested at pure mode I loading. On the other hand, the mirror-like fracture surface was observed for the HTHL specimen tested at pure mode I loading. Under pure mode II loading, the aligned stretched zone due to the maximum shear stress was the main characteristic of the fracture surface of LTLL specimen, whereas irregular appearance of the stretched zone was observed for the HTHL specimen. (c) 2008 Wiley Periodicals, Inc.