This study deals with the problem of identifying the member forces in a prestressed pin-jointed structure. For the purpose of evaluation of the identification accuracy, we first present the formulation of identification error, where measurement errors in member forces as well as nodal coordinates are incorporated. An optimization problem is then considered to search for the optimal set of locations for measurement of member forces, so as to have the highest identification accuracy. This typical combinatorial optimization problem is solved by the simulated annealing combined with a stingy method, and the accuracy and efficiency of the proposed method are demonstrated by numerical examples.