POLAR is a novel compact space-borne detector conceived for a precise measurement of hard X-ray polarization and optimized for the detection of the prompt emission of Gamma-Ray Burst (GRB) photons in the energy range 50keV-500 keV. GRBs are sudden flashes of gamma-rays that appear randomly in the sky and outshine for a few seconds in all other gamma-ray sources. They are produced at cosmological distances, and are considered as the brightest events in the universe after the Big Bang. In the past 40 years many instruments have performed extensive studies of GRBs, but their creation mechanism and their progenitors are still uncertain. Several theories have been elaborated to explain their origin: The fireball, the electromagnetic, and the cannonball models are at present the most commonly accepted. All of them relate the emission of the GRB to the creation of a black hole, differing in the physical processes involved in theg-ray generation, and also in the level of linear polarization of the emitted photons. The direction and the level of polarization of high-energy photons emitted by astrophysics sources such as GRBs are therefore very good observable candidates for the understanding of the corresponding emission mechanisms, source geometry and strength of magnetic fields at work. POLAR consists of a target of 40×40 plastic scintillator bars, each of dimension 6x6x200 mm3 , wrapped in a highly reflective foil, organized in 25 independent modular units, with 64 bars each. Each unit is read-out by a flat panel multi anode photomultiplier tube (MAPMT; H8500, Hamamatsu), mechanically coupled to the bottom of the scintillator bars via a thin transparent optical pad, and enclosed in a 1 mm carbon fiber socket. The electrical signals coming from the MAPMT are first processed by an ASICs and FPGA at the front-end electronics, then sent to the POLAR central computer, where the trigger decision is taken considering the outputs of all modular units. This modular design provides a good mechanical stability and facilitates the interchange of modules during the testing phase of the detector. The bars in each modular unit are kept together with two aligning plastic frames located at the top and at the bottom of the carbon fiber sockets to provide resistance to vibrations and to reduce the optical crosstalk between adjacent channels. Fig. 1 and Fig. 2 show the structure of POLAR whole detector and POLAR single module. The whole target, together with the central computer, the power supplies and the rest of the electronics, is further enclosed in a carbon fiber box that enhances the mechanical stability and acts as a shield against low energy charged particles. The POLAR detector will be mounted onto a satellite, to be able to study photons in the energy range between 50 and 500 keV, which cannot reach the ground because the Earth atmosphere absorbs all photons in the X-ray energy range. POLAR is an international collaboration project involving China, Switzerland and Poland, and has been launched onboard the Chinese space laboratory Tiangong-2 (TG-2) on Sep 15, 2016, under the support of National Natural Science Foundation of China (NSFC) and the Knowledge Innovation Program of the Chinese Academy of Sciences.