The measurement of worker differences is fundamental consideration in personnel assignment which is one key decision that influences the productivity and quality of assembly production. Due to the weakness of presented researches on worker differences measurement, the concept of station fitness, which takes the worker skill level and accumulated execution time in a given period as parameters, is proposed to better describe the worker competency and provide a way of measuring the differences among workers. A personnel assignment optimization model based on station fitness for assembly production is constructed, and the objectives are maximizing the station fitness for each assembly station and minimizing the difference of station fitness for workers assigned in the same assembly line. Then a heuristic algorithm based on the fitness matrix is presented to solve this model. The results from the example demonstrate the feasibility of the approach.