A method for measuring electric power value in a HVDC system comprises the steps of receiving current values and voltage values, which are measured from sensor group installed in specific positions of an electric power conversion station; identifying a line impedance of the positions where the sensor group are installed; calculating a first electric power value of the electric power conversion station; calculating a second electric power value of the electric power conversion station; calculating a third electric power value of the electric power conversion station; and determining one of the second and third electric power values as an actual electric power value of the electric power conversion station by comparing the calculated first to third electric power values with one another.