Worker assignment is one of the key factors which influence the productivity of assembly line. The concept of post fitness is presented for the description of the competency of assembly workers. The post fitness which takes the operator skill level and accumulated assembling time as parameters is formulated. An optimization model for assembly worker assignment is constructed, and the objectives are maximizing the post fitness for each assembly station and minimizing the difference of post fitness among operators working in the same assembly line. A heuristic algorithm based on the fitness matrix is presented. An example is presented for illustration and the results suggest that the approach is feasible.