A new method of detecting loss of synchronism between groups of generators is proposed. Suppose a group of generators swinging coherently losses synchronism against the remaining generators that also swing coherently. The loss of synchronism can be detected by using the relative phase angle α between current and voltage. The current is measured on a transmission line connecting two sub-networks to which these two groups of generators are belonged respectively and the voltage on a bus connected to the transmission line. Also, the magnitude of current flowing through the transmission line and that of voltage are measured and used for detection. When loss of synchronism takes place between two groups, the relative phase angle α exceeds 90 degrees. Also at the instant when α exceeds 90 degrees, following two conditions hold: the magnitude of voltage is near to the minimum value and that of current is near to the maximum value. These characteristics are used for the method. Theoretical background is explained first and the validity of the proposed method is shown by numerical examples using the model network EAST10.