The effect of shear flow on isothermal crystallization behavior of gamma-crystals in metallocene-based isotactic polypropylene melt was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD). In the sample under weak shear (at strain of 300% for 30 s duration), simultaneous evolution of alpha- and gamma-crystals occurred, and the final fraction of gamma-crystals (f(gamma)) was 0.66, which was identical to the undeformed sample (PP-Static). In this scenario, alpha-crystals probably served as effective seeds for nucleation of gamma-crystals. In the samples under strong shear (at strain of 500% for 30 s duration or long-time continuous shear at strains of 100% and 500%), the sequential emergence of alpha- and gamma-crystals was observed. In this case, molten polymer chains were probably constrained by the surrounding crystals after intense short-time shear and/or maintained their extended chain conformation after long-time shear. These oriented chains had little chance to form the gamma-crystals directly, behaving very differently from the relaxed chains. Under strong shear fields, the emergence of gamma-crystals was delayed or inhibited, whereas the f(gamma) value was also decreased rapidly. A simple model for the possible pathway of gamma-crystal formation in the strong shear environment was proposed.