Multi-sided assembly line is a typical production line in factories, where tasks are performed parallel in different sides of the assembly line. This type of line is normally found in producing large products such as cars. It is very important for the production line to be balanced in order to improve the production productivity. This paper presents a new approach based on simulated annealing algorithm to vertical balancing of multi-sided assembly lines. Zoning constraints, cycle time, working time and precedence relationships are considered as hard constraints while positional constraints are considered as soft constraints. To show the applicability of the proposed approach, it is applied on a real sample assembly line. In order to find the most suitable values for the parameters of the algorithm, different scenarios have been run on the sample assembly line. Findings indicate that the proposed approach is highly capable to achieve the predetermined goals of the line balancing problem.