The T91 heat resistant steels are welded by the laser-arc hybrid welding technique. The high temperature aging treatments of welded joints for different time at 750 ℃ are conducted and the microstructure evolutions of weld, heat affected zone and base metal are investigated. The corresponding precipitation is also analyzed. The results show that, with the increase of high temperature aging time, the grain size in the weld gradually increases, the width of the martensite lath increases and the martensite laths are gradually broken into subgrain structures. Simultaneously, the precipitation gradually coarsens and gathers to the grain or subgrain boundary and the micro-hardness gradually decreases. After the treatments for different aging time, the fractured positions of all welded joints are within the base metals.