Lattice steel transmission towers (LSTTs) are vital components of overhead transmission lines that play an important role in supporting the electrical power grids. This paper presents different types of failures observed during full-scale experimental testing on three LSTTs. The objective of this study is to check the absolute towers capabilities and to identify the strength and safety of the steel towers under the design loads. The full- scale tests were performed at CELPI towers testing station – Bucharest – Romania. The three LSTTs were designed to carry 33 kV double circuits overhead transmission lines, the loading conditions and design complied with the relevant requirements of the technical specification of ASCE-10/97. The structural design was performed using the software of power line system (PLS-TOWER) whereas the detailed design and drawings were performed using TEKLA STRUCTURE structural engineering software. The tests were performed in accordance with the requirements of the current version of IEC 60652:2002-06 “Loading test on line towers". Observed Failures were studied and the reasons are discussed in details. As a result, the modifications for each tower were performed in order to enhance the performance to sustain loads and decrease the overall deflection.