The effect of moisture absorption on fracture behaviors of acrylonitrile-butadiene-styrene (ABS) resin has been studied. For comparison, polystyrene (PS) and styrene-acrylonitrile (SAN) resin have been tested. The fracture toughness of PS and SAN resins is determined by the ASTM standard test method for brittle polymers. The fracture toughness of ABS resin is obtained on the basis of the multiple specimens method. The fracture toughness of PS and SAN resin decreases with the increase in moisture absorption. On the other hand, the fracture toughness of ABS resin slightly decreases despite enormous moisture absorption. On the specimens absorbing moisture, a bright whitening region and a milky coloring region are distinguished in the stress-whitening region, and the milky coloring region expands around the bright whitening region at the crack tip. From the transmission electron microscopic observation, the precraze formation carl be recognized in this region. The crack-tip shielding effect induced by this formation compensates the fracture toughness decrease due to moisture absorption.