The search for a light sterile neutrino started to be a popular topic in neutrino physics, since the observation of the "gallium anomaly" and "reactor anomaly" in which a deficit of neutrinos was observed relative to the prediction. Such anomalies could be explained by short distance oscillations towards a sterile state with Δm2 ∼ 1eV2. In a highly competitive context, several projects were started to search for a light sterile neutrino. This paper will focus on the STEREO experiment for which a detector has been designed to observe the electron antineutrino energy spectrum distortion from 3 to 8 MeV due to such a new L/E oscillation, and should therefore confirm or reject the light sterile neutrino hypothesis. Electron anti-neutrinos produced by the compact reactor core of the Institut Laue-Langevin will be detected in a 6-cell segmented volume of Gd-loaded liquid scintillator through the inverse β-decay process. The STEREO detector was completed in November 2016 and was commissioned right away, STEREO is currently taking data. In this paper we will present the final design of the detector and the first results of the commissioning.