•The creep void turns to be the leading factor in the creep/fatigue accelerated failures.•The dendrite separation and γ' phase rafting play as the dominant microstructural behaviors.•A ‘void migration mechanism’ develops in the creep/fatigue acceleration tests.•The failure of turbine blade changes from the mixed to the intergranular mode.•The phenomena are essentially controlled by the contributions of creep and fatigue damages. Creep/fatigue accelerated failures of K403 superalloy turbine blades are inspected, and the damage behaviors are revealed and failure modes are determined in laboratory. It is concluded that: (1) significant discrepancies of fractographies and cross-sectional microstructures are interpreted by qualitative and quantitative investigations; (2) the observed morphologies are mainly attributed to the dendrite separation and γ' phase rafting behaviors, together with the development of ‘void migration mechanism’, i.e., grain interior → grain boundary → sub grain boundary; (3) the failure of turbine blade changes from the mixed to the intergranular mode, which is essentially controlled by the contribution proportion of creep damage and fatigue damage.