The Central dispatch center tried to close 70-71-0 section CB(Circuit Breaker) to send the power produced by #3,4,5,6 generators through 345kV Asan-Boryung#1 and Boryung-Boryung CC(Combined Cycle) transmission lines at 10/16/2010. When 70-71-0 section CB was closed, the power outputs of #3,4,5,6 generators start to oscillate and the maximum power output reached up to 600MW at each generator. In this paper, we will analyze the power oscillation of #3,4,5,6 Boryung generators when 70-71-0 section CB is closed. We will simulate this phenomenon using PSS/E developed by SIEMENS. In addition, this paper will provide how to prevent power oscillation and connect 345kV Boryung 70 - 71 bus and 75-76 bus safely.