This paper deals with a transfer line optimal design. In transfer lines, operations of the same block are executed simultaneously. Blocks are assigned to machines and they can be activated in mixed order. The set of all available blocks is given beforehand. The line investment cost is defined by the sum of blocks costs and stations costs. In addition to the standard line balancing problem, precedence and cycle time constraints, blocks compatibility and parallelism constraints must be taken into account. The problem is to assign all operations grouped into blocks that all constraints are respected and line investment cost is minimum. This paper is focused on solving the problem by a branch-and-bound algorithm. A new approach for obtaining a lower bound is offered. It is based on a reduction of the transfer line balancing problem to a set partitioning problem. Computational experiments provides that the proposed approach is efficient to solve practical transfer line design problems