Although load's dynamic behavior is well known to affect power system stability significantly, load's dynamics itself is not well known. As a result, power system stability assessment is very far from sufficient accuracy even today in 21^st century. Structure of existing power system load is assumed to be parallel composite of induction motor (IM) and constant impedance (CZ) behind a series reactance. Three parameters: IM ratio, inertia, and loading are identified by measured data using least square error method. The series reactance is identified by system aggregation method considering temporary system structure, and about 17% at peak load demand in studied 9 pure load areas. Effects by temporary included local generators can be corrected. Using 594 measured data of voltage sag accompanying network faults, the three parameters: IM ratio, inertia, and loading are identified as 55%, 0.6 second, and 70% respectively at summer peak demand when the voltage stability will become the worth. The parameters are slightly affected by season, demand, and area. All three parameters very slowly decrease by year.