Stability of plastic deformation and toughness of bisphenol-A polycarbonate (PC) blended with poly(acrylonitrile-butadiene-styrene) (ABS) copolymer have been studied by three-point bending tests of U-notched bars under plane strain state. The deformation of PC/ABS blends was transformed into a stable mode by depression of craze nucleation when the content of modifier was increased beyond a certain critical level which was dependent on the cohesive strength of the ABS modifier. It was concluded that the deformation mode of PC/ABS blends could be explained in terms of the theory of instability of plastic deformation in the region of pre-formed voids under dilatational stress. Furthermore, toughness improvement was found to be very sensitive to the decrease of cohesive strength of the modifier and was efficiently accomplished by the addition of a small amount of the modifier of low cohesive strength.