When multiple flaws are detected in structural components, the remaining lives of the components are estimated by fatigue flaw growth calculations using combination rules in fitness-for-service (FFS) codes. Many FFS codes provide combination rules for multiple flaws; however, these rules differ significantly among the various codes. Fatigue flaw growths for two similar adjacent surface flaws in a flat plate subjected to a cyclic tensile stress were obtained by numerical calculations using these different combination rules. In addition, fatigue flaw growths taking into account the interaction effect between the two similar flaws were conducted by the extended finite-element method (X-FEM). The calculation results show that the fatigue lives calculated by the X-FEM are close to those obtained by the American Society of Mechanical Engineers (ASME) Code. Finally, it is noted that the combination rule provided by the ASME Code is appropriate for fatigue flaw growth calculations.