Carbon fiber reinforced polymer (CFRP) has great potential for strengthening aging steel structures, compared to traditional methods of attaching steel plates which are often bulky, heavy and difficult to fix. CFRP provides advanced structural properties with light weight and high tensile characteristics, making it an attractive option to perform the aforementioned role. This paper presents a comprehensive experimental program to investigate the torsional behaviour of strengthened steel square hollow sections (SHS) and strengthened aluminium SHS with various thicknesses. From previous research, a wrapping angle of 45° is the most efficient in terms of increasing the maximum torsional capacity, and was adopted for the tests. It was found that sections with larger depth to thickness ratio (D/t ratio) had a greater percentage increase in maximum torsional capacity. Moreover, it was observed that sections with lower elastic moduli compared with steel will show more improvement in maximum torsional capacity for a given thickness.