Abstract The aim of this work is to investigate the temperature evolution, microstructure and mechanical properties of the joint during friction stir welding of thick 2219-O aluminum alloy with the butt joint. Different types of thermocouple layouts were designed to measure the temperature evolutions during tool inserting and stable welding processes. Dynamic recrystallised structures in the weld nugget zone (WNZ) were observed and different grain sizes were presented through the thickness. Tensile testing results indicated that the tensile strength ( σ b) and yield strength ( σ 0.2) of joints increased and the elongation ( δ) of joints decreased with increasing the tool rotary speed. The joint top had the highest mechanical properties. Most of the failures presented a ductile fracture in the joints. Vickers microhardness was measured across the joint top, middle and bottom, and through the thickness in the WNZ. The result showed that the microhardness of different zones varied significantly.