Abstarct: Surge arresters are a protective and strategic device against transient overvoltage in the power system. Their inappropriate and untimely performance causes heavy damage to high voltage devices. Therefore, it is very important to diagnose and evaluate their condition in the operating period. According to the operating conditions, the leakage current of the surge arrester changes. Consequently, the analysis and evaluation of the leakage current provides useful information about their status and situation, which can be used to monitor this equipment. In this thesis, the analysis and evaluation of the leakage current of the mextal oxide surge arrester has been investigated. In order to consider the effective factors on the leakage current, the effect of uniform and non-uniform contaminations (fan shape type) under different humidites have been evaluated in the laboratory environment. Fan shape type non-uniform contamination at two angles of 90 and 180 degrees has been investigated and the harmonic components of the leakage current have been separated using FFT. The results obtained from the experiments show that the leakage current and its harmonic components increase with the increase in humidity and the equivalent salt deposit density (ESDD). Meanwhile, increasing the degree of non-uniformity of pollution on the surface of the surge arrester has a decreasing effect on the leakage current and its harmonic components. In order to investigate the effect of non-uniform pollution on surge arrester monitoring indicators, ir3/ir5 and ir1/it1 indicators have been evaluated. The obtained results have shown that the main component of the resistive leakage current increases greatly with the increase in the surface pollution laxyer ESDD at uniform condition. baxsed on this, the ir1/it1 index increases. Examining the results in non-uniform pollution conditions indicates that with the increase of the non-uniform pollution degree, the percentage of ir3 reduction will be higher than ir5, so the ir3/ir5 ratio will decrease. Also, the non-uniform degree of pollution has a greater effect on ir1 than it1, for this reason, with the increase of the non-uniform degree, the ir1/it1 index decreases. In general, although the investigated indicators under uniform pollution conditions can be used to monitoring of the surge arrester, these indicators are strongly affected by non-uniform pollution and the threshold limits defined for them will change.