This paper addresses the compensation problem of the missile radome aberration error using the interacting multiple model (IMM) algorithm. To compensate the radome error, the proposed method estimates the radome slope by applying the modified IMM algorithm. In the proposed approach, the mode probability update in the IMM algorithm is redefined for improving performance of the radome slope estimation by reflecting the recent tendency of the radome slope hypotheses. This is enabled by utilizing the new weighting function including previous information of likelihood function and mode probability during certain period. Besides, by designating boresight error and LOS rate as the state for the system model, the linear estimation technique is used instead of the nonlinear estimation method. Therefore, the proposed method is suitable for the real-time implementation due to the linear filter structure. Through the simulation results on the typical engagement missile-target scenario, the reliability and performance of the proposed radome slope estimator is evaluated.