GaAs is a widely used III-V semiconductor compound in microelectronics. Since the 90's, a growing interest is observed for its larger integration in electronics industry (micro and nano electronics) and solar in cells. Therefore, new characterization needs are emerging to address this kind of applications. In particular, specific features such as the thermal properties are of great interest. In this framework, the thermal conductivity of GaAs substrate has to be studied. In this paper, we present the results obtained when characterizing a 350 mu m thick GaAs substrate by the so-called 3-omega method. This latter has been widely exploited to determine the thermal conductivity of a large range of materials. It makes use of a thin metal strip, deposited on the sample under test, which serves as both a heating element ant a sensor. The results obtained experimentally are compared to those issued from an analytical solution (Cahill solution).