In this research, a numerical investigation of the thermal behaviour around a circular planar micro-coil with a variable width, deposited on a Kapton substrate has been presented using finite element method. The purpose of this study is to provide a thermal approach that takes into account the effect of air buoyancy which has not ever been used in this field. Moreover, the influence of this micro-coil inner width on its electrical and thermal behaviour has been studied. The width of the micro-coil decreases from the value of w i to w (w ⩽ w i ) with 100 ⩽ w ⩽ 600 µm. The three-dimensional simulation model focuses on the Maxwell's equations coupled to the momentum and heat equations. The air buoyancy is introduced along the vertical direction in the momentum equations when the micro-coil is considered as an internal heat source. In a first step, the effect of air buoyancy on the thermal profile has been visualized to highlight the changes that can occur in terms of temperature distribution. The results obtained have shown that the electromagnetic characteristics of this type of coils are improved according to the decrease of the width (w). However, the temperature increases significantly, which tends to accelerate the air motion and favors the convective heat flow. It should be noted, that the study of this kind of phenomena permits to identify the thermal operating zone of insulating materials, practically the Kapton and to prevent thermal damages that may occur.