Highly ordered As/Si faceted lateral nanoprisms array with a uniform width of about 10-18 nm are fabricated on Si {1 0 3} miscut surface via thermal deposition of sub-monolayer arsenic followed by a subsequent annealing process. In situ scanning tunneling microscopy (STM) studies prove the compactly arranged As/Si nanoprisms orienting along <3 0 1> direction, which are bounded by As terminated {1 1 3} facets with 2 x 2 reconstruction. The formation mechanism of the nanoprisms and the surface faceting is investigated via surface energy and chemical potential comparison for the As/Si systems. Ab initio calculations attribute the driving force of the geometry transition to the minimization of surface energy. Moreover, the compatibility of the lateral As/Si nanoprisms with the well-established Si technology propose an alternative strategy to fabricate III-V nanowires on Si substrate, which is promising for both the fundamental research and a variety of applications.