This invention relates to a method of determining stability of unstable equilibrium point (UEP) computed by using BCU method, comprising selecting UEP computed by using BCU method, obtaining a test vector X.sup.test for the selected UEP, say X.sup.UEP using the following equation: X.sup.test=X.sub.s.sup.post+0.99(X.sup.UEP-X.sub.s.sup.post) where X.sub.s.sup.post is the SEP, and checking boundary condition of X.sup.UEP by simulating system trajectory of post-fault original system starting from X.sup.test.