The interaction behavior of two non-aligned through-wall cracks in flat plates is investigated by the finite element method (FEM) under extensive creep condition. The time-dependent fracture parameter C*-integral along the crack tips are calculated and compared to the results of a single crack of the same size. For comparison purpose, the interaction of stress intensity factors (SIFs) is also examined in the study. The results indicated that interaction of multiple cracks is different between the time- dependent fracture characterized by C*-integral and linear elastic fracture noted by SIF. The magnifying factors of time-dependent fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current re-characterization rule for multiple cracks developed from linear elastic fracture analysis may lead to a non-conservative result and should be modified when it is used in the assessment of time dependent failure.