The present work concerns the moisture effect on electrical features of insulating paper. Despite the progressive replacement of paper by different materials, several blocks and other insulating screens in transformers contain cellulose which is characterized by a very high hygroscopicity. These cellulosic products appear as a source of moisture. This moisture can lead to a serious damage in the electrical equipments. The aim of this work was also to focus our attention on examination of dielectric losses evolution with the moisture level. For a better understanding of phenomena, we have separately examined the moisture influence on both conductivity and permittivity of the paper.