In this paper, we present a new mathematical model of U-type assembly line balancing problem based on the performance factors of the operators, the amount of energy used and incompatibility between the tasks. Many researchers worked on U type line balancing problems are focused on minimizing of number of workstations. However, this may cause overloaded work assignments to workers. Excess workload may cause fatigue and carefulness and consequently output rate of the line fluctuates, quality problems arise, and risk of work accidents is increased. In order to tackle with the mentioned problems and to provide the stability, an integer programming model that includes some ergonomic factors is proposed. In this study, model versions are examined by considering ergonomic factors on test problems from the literature. We conclude that the suggested model may be very useful to make decisions on allocating tasks to workers in U-type assembly lines.