To improve solution efficiency and automation of assembly sequence planning, a symbolic ordered binary decision diagram (OBDD) technique for assembly sequence planning problem based on unordered partition with 2 parts of a positive integer is proposed. To convert the decomposition of assembly liaison graph into solving unordered partition with 2 parts of positive integer N, a transformation method from subassembly of the assembly to positive integer N is proposed, the judgment methods for the connectivity of a graph and geometrical feasibility of each decomposition combined with symbolic OBDD technique is proposed too, and all geometrically feasible assembly sequences are represented as OBDD-based AND/OR graph. Some applicable experiments show that the symbolic OBDD based algorithm can generate feasible assembly sequences correctly and completely.