Abstract: Since long time ago, ceramic insulators are widely being used in power systems. During operation, the insulators may severe a certain degree of pollution, which may reduce their performances such as surface resistance and flash over voltages. The pollution may arise in the form of fogs. A large Leakage Current (LC) may flow on the surface and degradation may take place. In long term the degradation may lead to the flash over of the insulators. This study reports the experimental results on the effects of humidity and fog conductivity on leakage current characteristics of ceramics. The samples used are blocks of ceramics with dimension of 250x50x20 mm³. The samples were made from same materials for outdoor ceramic insulators. The samples were put in a test chamber with controlled humidity and artificial pollution conditions. AC voltage with frequency of 50 Hz was applied. The tests were conducted according to IEC 60-1 and IEC 507 (fog test). The magnitudes as well as harmonic content of the LC were analyzed. The experimental results indicated that under fog condition, LC magnitude increase with fog conductivity. At high fog conductivity the oscillation behaviour of LC magnitude dependence on applied voltage was observed due to the wetting effect of sample surface by the fog and drying effect caused by the higher LC. The THD of leakage current under salt fog increased with the applied voltage. LC amplitude increases with applied voltage and RH. LC waveforms distortion significantly decreased with the salt fog conductivity resulting in smaller THD. At given applied voltage the harmonic components decrease with the increase of fog conductivity.