The deformation behavior of poly(styrene-b-butyl methacrylate) diblock copolymers, PS-b-PBMA, is studied by high-voltage electron microscopy (HVEM) with in-situ deformation device and by TEM. This allows us to describe the craze growth and propagation in block copolymers with different morphologies. The influence of shape and orientation of morphology on deformation mechanism is discussed in correlation with tensile properties. For lamellar, hexagonal, and lamellar/hexagonal structures diversion of crazes and craze stopping mechanisms are observed. The discussion of the dependence of craze initiation stress, sigma(c), on morphology is used to correlate the micromechanical deformation processes with tensile properties. In contrast to other block copolymers, it is shown that sigma(c) exceeds the value of pure PS at 76% PS due to the hexagonal morphology, high miscibility, and broadened interface of PS-b-PBMA which explains the improved tensile properties of weakly segregated block copolymers. It is shown that microphase separated morphologies, phase behavior, and interface formation have a pronounced influence on deformation behavior of PS-b-PBMA diblock copolymers.