Overvoltage in power system is frequent phenomenon system which is not possible to avoid, but, damaged caused by it can be certainly controlled. Lightning Arresters are installed in the system for the overvoltage protection of transmission and distribution system. During normal operating condition Arrester acts as an insulator whereas, it offers conductive path during overvoltage. Hence, it is a core protective device for transmission & distribution system against lightning over voltages. It consists of zinc oxide (ZnO) elements so called as metal oxide surge arrest (MOSA). Due to ageing effect MOSA degrades under influence of continuous operating voltage, pollutants and surge voltages. Degradation of MOSA increases the leakage current flowing through it which results increased in temperature of surge arrester. The resistance of zinc oxide elements decreases as temperature increases. As a results leakage current increases, which again increase the temperature of a MOSA. This results in to thermal runaway conditions for MOSA. After thermal runaway condition it does not return to normal condition. This condition is basic reason for failure of MOSA. So, condition monitoring of surge arresters should be done at periodic intervals. Various Offline and online condition monitoring techniques are available for health assessment of MOSA. Offline condition monitoring techniques has main drawback that we need to remove surge arrester from the system, so system shutdown is required. As no need to take system shutdown online condition monitoring techniques preferred. ERDA has expertise of various condition monitoring techniques of MOSA and conducted online health monitoring of various ratings of MOSA up to 400 kV. The amplitude of total leakage current (IT) and amplitude of resistive leakage current (IR) have been considered as indicators for surge arrester condition monitoring. This paper presents ERDA’s experience of the condition monitoring of MOSAs with case studies.