Low temperature will affects the mechanical properties of angel steel transmission tower and weaken its static and dynamic load resistance ability, which may lead to brittle fracture of tower member. Angle steel leg member is the major stress components of transmission tower. Once the leg member brittle failure occurs, it will endanger the operation of the transmission line and the entire power system. In this paper, the low temperature uniaxial tensile test and Charpy impact test are conducted to investigate the low temperature mechanical properties of Q345B and Q420C angle steel as well as its welded joint with various thicknesses.The results show that the ductile brittle transition temperature of Q345B angle steel and its welded joint，as well as Q420C angle steel and its welded joint are -2.59, -15.28, -32.33 and -6.76 ℃ respectively. The tensile strength and yield strength of four kinds of steel will be increased with temperature. In the cold area which temperature reach -45 ℃ or below, Q420C angle steel should be used in transmission towers to meet the design requirements, however the welding process of Q420C angle steel should be avoid.