The AC leakage current of surge arresters is the most important information for monitoring and the condition assessment process. The ambient humidity can result in the increase of the leakage currents and the variation of harmonic components. The aim of this study is assessment of 20 KV brand new polymer surge arresters in humidity conditions. This paper described leakage current characters of 20 KV polymer housed metal oxide surge arrester in different humidity conditions and voltage levels by using experimental tests. The characteristics of the leakage currents have been investigated under different applied voltage when changes the ambient humidity in an artificial fog chamber. A joint approach of Fast Fourier Transform (FFT) amplitude is used to analyze the leakage currents in combination with the leakage current waveform. Also this paper presents the analysis of the harmonic components of leakage current in different conditions. Finally, a good indicator is introduced for assessment of polymer surge arresters in humid ambient conditions.