When applying area method to estimate dynamic stability for power system and hold system stability with having a necessary reserve, protection devices have a short time to caculate and time cutting exactly (follow angle cutting). There are some methods to do this task but there is not a conclusion to show optimal option. By the way, authors propose building a program to to investigate stability using Matlab software to calculate faster, bring out some exact decisions to have an ensured rescue power system out of losing stability. In addition, it helps data synchronous process faster in power dispatch working.