ATP(Alternative Transients Program) is a royalty-free EMTP (The Electromagnetic Transients Program). ATP and EMTP are considered to be one of the most widely used software for the analysis of power systems now in the world. The steady state and transients analysis of voltage, current, power flow and so on is possible. It has many models including rotating machine, transformer, surge arrester, transmission line and cable. The analysis of control system is also possible by using TACS and/or MODELS. The recent improvements of switch logic enabled the correct simulation of power electronic systems.