Abstract The increasing need to improve security monitoring of networks by system operators is the main motivation of the work described in this paper. The paper provides an algorithm for the on-line probabilistic transient stability assessment of existing or forecasted operating conditions. The proposed algorithm is an analytical method making use of two types of information: (i) transient stability assessment tool, and (ii) probabilistic factors. The corrected transient energy margin (CTEM), which is a hybrid method, is used to on-line transient stability assessment. The probabilistic factors of the conditional probability theorem were included in our method. Hence, our algorithm calculates the probability of the transient instability through using the CTEM and probabilistic factors in the transient stability in an on-line manner for exciting or forecasted operation condition for short term operation condition (e.g., next 1 h period). The implementation of the proposed method to New England 39-bus test system showed the ability and good performance of the method for the probabilistic assessment of the transient stability. Also, the method provides the system operator with a profound insight into the system stability conditions. The results retrieved again with changed operation conditions; therefore, the system operator can sometimes improve the system stability condition through making little changes in the power output of generators.