The mechanical toughness of modified ABS (acrylonitrile-butadiene-styrene) alloys was evaluated using Izod impact, tensile, and compact tension tests. The modified ABS alloys contain 20 wt % of styrene-N-phenylmaleimide (SMI) that is added to enhance the thermal resistance of the ABS. In this study, the effects of matrix composition, rubber/ matrix adhesion, and rubber particle structure on the alloy toughness were investigated. Results from the tensile test and Izod impact test ranked the alloys in an order that is different from that given by K-Ii (stress intensity factor for crack initiation), measured from compact tension specimens. This is due to the difference in energy-absorption characteristics for crack initiation and crack growth. The conclusion is supported by optical micrographs on the deformation zone size. The microdeformation behavior of the alloys was examined using transmission electron microscopy (TEM), which revealed different rubber toughening mechanisms between Izod and tensile specimens. The former contains numerous extensive crazes, while the latter, only a very few short crazes, except in regions within a few micrometers from the fracture surface. The dominant matrix deformation mechanism for the tensile specimens is believed to be shear deformation. Another interesting observation from the study is rubber particle cavitation, commonly observed in tensile specimens and Izod specimens with solid rubber particles; it did not occur in the Izod specimens containing salami-type rubber particles. This is attributed to the salami structure that increased the straining rate for the rubber phase, leading to ductile-brittle transition of the rubber. The transition to brittle deformation of the rubber phase prevented rubber particle cavitation. The microscopic examination indicated that toughening mechanisms by the rubber particles can be very different among the mechanical tests, which should be taken into account for the rubber toughening of polymers.