In this research, we address U-type assembly line balancing and aim to solve large scale instances to optimality. Specifically, we consider simple line balancing problem with minimizing the number of workstations (UALBP-1) and cycle time objectives (UALBP-2). Optimal solution of UALBP-1 is important, since each additional station requires additional workers and equipment. On the other hand, for UALBP-2, a slight improvement in cycle time increases the production capacity. To be able solve large scale instances to optimality, a decomposition based algorithm is proposed and enhancement strategies are integrated. We perform computational experiments to test the efficiency of the algorithm and present the results. The main contribution of this paper is the proposed decomposition strategy and integrated acceleration mechanisms.