This letter presents a simple method to suppress the sidelobe level of stub-loaded endfire leaky-wave antenna (LWA) by dividing the original uniform one into two segments and employing different sizes of elements in each part for different equivalent normalized phase constants in the operation band. Since the phase constant of the LWA determines the main beam angle as well as the angle positions of poles and nulls in the pattern, the pattern poles from the first segment can be properly changed and located at the pattern nulls of the second one with some relation between two parts so as to realize the sidelobe suppression for the whole antenna. A radiation model is provided to reveal the effects of power ratio, period length ratio, and phase constant differences between two segments on the sidelobe level of LWA so as to determine the required element numbers and lengths in two parts. A prototype antenna is in finally designed and fabricated to verify the predicted performance of the proposed LWA. Good agreement between simulation and measurement validates the effectiveness of the proposed method on the sidelobe suppression for the stub-loaded endfire LWA.