This paper presents a new optimization tool for automated design of steel lattice transmission line towers in real-world engineering practice. This tool has been developed by integrating the simulated annealing (SA) optimization algorithm into the commercial PLS-TOWER software to optimize steel lattice towers for minimum weight according to ASCE 10-97 design specification using both size and layout design variables. In this context, a novel two-phase SA algorithm is specifically developed and compared with a typical SA formulation in three weight minimization problems of real-world steel lattice towers for high voltage overhead transmission lines between 110 and 400 kV. The optimized designs and the CPU time required by the two SA variants are reported for each test problem and then compared with the currently available structural configurations resulting from a conventional design process in order to quantify material saving achieved through optimization. (C) 2016 Elsevier Ltd. All rights reserved.