This paper describes the development of the transient analysis model of 345[kV] 100[MVA] STATCOM and the result of the performance analysis. The Korea power utility company, KEPCO began a commercial operation of a 100[MVA] STATCOM(Static synchronous COMpensator) at 345[kV] Migeum substation in Seoul. In order to operate a new control device like STATCOM to the power system, suitable transient model is needed. Therfore, Korea Electric Power Coporation(KEPCO) developed a transient analysis models using EMTDC/PSCAD. The development of this transient analysis model is based on the actual STATCOM S/W algorithm and H/W specification that was provided by its manufacture. Through this model, the transient/dynamic performance of STATCOM at Migeum substation can be analyzed.