The incremental dynamic analysis (IDA) by the concept of equivalent single degree of freedom (ESDOF) is applied to the bridge structures in this study to observe the effects of two different lateral load pattern, i.e., 1st model load pattern (1st) and Uniform acceleration load pattern (Unif), on the seismic behavior of the RC bridges under three ground motions generated corresponding with the design spectrum for the inner area of Bangkok, Thailand. Three different bridge's column heights were considered to investigate the effect of the different substructure flexibilities on the accuracy of using IDA by ESDOF to evaluate the seismic behavior of the bridges. The results show that the different lateral load patterns influences on the different lateral capacity of the bridge especially when the rotational mass moment of inertia was considered. The different lateral capacity leads to different incremental dynamic analysis curve (IDA curve). The IDA curves show that the ESDOF with lateral capacity from Unif give slightly stiffer seismic behavior than the nonlinear time history analysis of whole bridges while the ESDOF with lateral capacity from 1st give significantly weaker for the bridge with short column height. However, the effect of different lateral load pattern on the IDA curves decreases when the bridge's column height increases.