This paper analyzes the existing performance of insulator strings described in term of the total flash over rate (TFOR) for 69 kV and 115 kV subtransmission lines in a service area of Metropolitan Electricity Authority (MEA), Thailand. Presently, the design and installation of insulator strings for 69 kV and 115 kV subtransmission lines in MEA are based on American National Standards Institute (ANSI). The applications of this standard may not be appropriate for insulation coordination evaluation in MEA because of the different number of thunderstorm days. As a result, the failure on insulator strings of subtransmission system in MEA was often found. The resultsshow that the difference in TFORs of the lines is significant for different thunderstorm days and probability peak current curves.