Determination of nonlinear dynamic behavior of structures has always been one of the main goals of both structural and earthquake engineers. One of the newest methods for analyzing seismic behavior of structures is Modal Incremental Dynamic Analysis (MidA). In fact, this method is an alternative to the Incremental Dynamic Analysis (IDA), which is a dificult and time-consuming method. Despite the MidA's approximate results, advantages such as adequate accuracy, high speed, and low cost make this method an efficient and appropriate approach. In all the previous studies, the proposed models have had a regularized plan; hence, all the analyses have been carried out on a frame. In this study, the MidA analysis was developed in an asymmetric-plan-type building by considering three structures with 4, 7, and 10 stories having irregularity in plan. Accuracy of the work was examined. Also, through a simplification, instead of considering an unconventional plan, we used a rectangular plan with an eccentricity of 15% between the center of mass and the center of rigidity. Comparing the results of this study and the IDA method proved the high level of accuracy of this method in assessing seismic demands.