As it is known that the increasing of harmonic currents levels on distribution system have effected on the cable current capacity. Harmonic current orders will cause increasing in the temperature of the conductor inside the cable which may exceed the limit value. The goal of this paper is to study thermal analysis of distribution cables to calculate the temperature distribution inside the cable layers and its surrounding soil in the presence of harmonic current order under daily load current cycle, taking into account the phenomena of a dry zone formation. Consequently, a de-rating factor is calculated in this study due to the harmonic currents effects. The thermal model of the cable is considered according to IEC 60853-2 standard, including the effects of current harmonics on temperature of the cable parts and its surrounding soil. Additionally, the finite element method is used to obtain temperature maps of the cable and the around soil. Calculations showed that the time of dry band creation is decreased when the level of harmonic increases in the loading current cycle. This study is done on the distribution system cables have voltage level 0.4 kV and 22 kV.