Build orientation is one of the most important process planning tasks in additive manufacturing (AM) since it directly affects the part quality, build time, cost etc. Many researchers have investigated the orientation optimization problem and proposed numerous solutions. However, former researches only focused on how to find an optimal orientation for one part, but none of the solutions was provided to solve the orientation optimization problem of Multi-part production , where a group of parts in the same build vat or chamber should be optimally-orientated simultaneously. This paper introduces a two-step solution to solve the problem. At first, a feature based method is used to generate a set of finite optimal alternative orientations for each part within a given part group to guarantee each part's individual build quality; then an improved genetic algorithm is applied to search for an optimal combination of part build orientations to minimize the total build time and cost at a global optimal level. A case study of orientating 16 parts simultaneously within a given build chamber is presented for demonstration.