This paper theoretically and experimentally investigates the inertial effects of the flap body on the performance of a bottom-hinged oscillating wave surge converter (BH-OWSC). A two-dimensional (2D) hydrodynamic theory for a BH-OWSC based on the assumption of potential flow is developed to show that one simple but critical parameter, i.e., the square of sum of three mechanical-impedance terms associated with the inertial effects, can precisely characterize the performance trend of a BH-OWSC. Model testing in a small-scaled wave basin follows to validate the theoretical formulations with a flap body consisting of multiple hollow cylinders into which water can be filled individually to alter the values of flap's inertial parameters. The performance of each inertial specification of the flap model is evaluated based on the measurement of the mean water discharge from the hydraulic pump (or the power take-off). Finally, the “near resonant condition” has been validated experimentally by altering the inertial parameters of the flap. Thus, the aforementioned parameter is shown to be capable of characterizing the inertial effects on the performance of a BH-OWSC, and the minimization of it will maximize the power capturing performance of a BH-OWSC. Consequently, the parameter can be used for design guidelines of the flap body in its inertial aspect, such as locating the center of mass and determining the geometric dimensions of a flap body.