A method of detecting out of step in an electric power network making use of a protection device having means for measuring vottage, (V A, V B) and current signals, (I A, I B) of the electric lines at both local and remote terminals of the electric network and having a computer means for data processing and calculation of an active power and reactive power at both the local terminal and at the remote terminal, the method further comprises the following steps: - calculating an apparent power phasor S A at the local terminal and an apparent power phasor S B at the remote terminal, based on the calculated active power and reactive power, - determining a first criterion Crit1 for out of step detection on the base of the behavior of the apparent power phasor S A and the apparent power phasor S B at the same time instant t k in the following form: Crit 1 = = 1 ⇔ S B crossess S A where: S A - is an apparent power phasor in local terminal, S B - is an apparent power phasor in remote terminal, - setting the threshold value t thr for the time in which the apparent power phasor S B crosses of the apparent power phasor S A or for the time in which the apparent power phasor S A crosses of the apparent power value S B, - determining a second criterion Crit2 for out of step detection which has a form: Crit 2 = = 1 ⇔ special timer t d = t thr AND S B not crossess S A after t d or Crit 2 = = 1 ⇔ special timer t d = t thr AND S A not crossess S B after t d where: t(d) means time duration of special timer, t thr means a threshold time value for crossing time, t d means a time instant of detection of the first criterion, - determining out of step presence by making a logic product of the results of the criterion Crit1 and Crir2 for each sample of interest at the same time instant, - automatically creating a signal for activating remedial action, triggering alarm or passing the signal indicating the out of step detection to other protection devices. - calculating an apparent power phasor S A at the local terminal and an apparent power phasor S B at the remote terminal, based on the calculated active power and reactive power, - determining a first criterion Crit1 for out of step detection on the base of the behavior of the apparent power phasor S A and the apparent power phasor S B at the same time instant t k in the following form: Crit 1 = = 1 ⇔ S B crossess S A where: S A - is an apparent power phasor in local terminal, S B - is an apparent power phasor in remote terminal, S A - is an apparent power phasor in local terminal, S B - is an apparent power phasor in remote terminal, - setting the threshold value t thr for the time in which the apparent power phasor S B crosses of the apparent power phasor S A or for the time in which the apparent power phasor S A crosses of the apparent power value S B, - determining a second criterion Crit2 for out of step detection which has a form: Crit 2 = = 1 ⇔ special timer t d = t thr AND S B not crossess S A after t d or Crit 2 = = 1 ⇔ special timer t d = t thr AND S A not crossess S B after t d where: t(d) means time duration of special timer, t thr means a threshold time value for crossing time, t d means a time instant of detection of the first criterion, t(d) means time duration of special timer, t thr means a threshold time value for crossing time, t d means a time instant of detection of the first criterion, - determining out of step presence by making a logic product of the results of the criterion Crit1 and Crir2 for each sample of interest at the same time instant, - automatically creating a signal for activating remedial action, triggering alarm or passing the signal indicating the out of step detection to other protection devices.