The big earthquake 28th March 2005 occurred at Indian Ocean near Sumatra Island. This area is known as one of the world’s most seismically active regions. The epicenter of this earthquake is surrounded by 2 plates, south (Indo-Australian plate) and north (Eurasia plate). It was recorded by seismic waves that the earthquake had magnitude 8.7 (USGS). In this paper we calculate the slip amount of the fault motion by using far field tsunami waveforms recorded at three tide gauges, Male, Cocos, and Colombo. Form the slip amount, the moment magnitude of the earthquake is determined. Comparison of numerically computed tsunami waveforms and observed tsunami waveforms revealed that the average slip amount of the fault motion is 4.7 m. Then, seismic moment (Mo) is 9.39x1021Nm. This result has similarity value as reported by HARVARD-CMT that is 11x1021Nm. The calculated moment magnitude (Mw) is 8.6 where this value is quite similar as reported by USGS that is 8.7.