Abstract　 　　 

　With the present state of exoplanet detection techniques, none of the rocky planets of the Solar System would be detected and indeed their presence is a very strong constraint on the formation scenarios of planetary systems. By measuring the reflex effect of planets on their host stars, astrometry can provide us the planetary masses and their informative orbits. This technique is used frequently and is very successful to determine the planetary masses and the orbits of binary stars. However, it is necessary to go to space to reach the precision required to detect all planets down to the telluric regime. Search for Terrestrial Exoplanets (hereafter STEP) mission is a latest advanced research project in Chinese Strategic Pioneer Program (SPP) on Space Science. STEP aims at the detection of earth-like planets about nearby stars based on the space-based high precision of astrometry with 0.5～1 uas, which will produce fruitful achievements in the exoplanetary and astrometry fields, to provide a comprehensive  understanding of the planetary systems.