Abstract: Laser beam welding of ZL109 aluminum silicon alloy was carried out by CO₂ laser without any filler,and microstructure and mechanical properties of the joints were investigated. The results show that weld microstructure consists of cellular-dendrite and dendrite crystals,and its phase composition contains α-Al,Si and Mg₂Si phases. The microhardness of weld is higher than that of both heat-affected zone and base metal,its average microhardness being 125 HV0.05 with the welding speed of 3.0 m/min. The heat input has a significant influence on mechanical properties of the joints. With the increase of heat input,both tensile strength and elongation of the joints increase firstly and then decrease gradually,the maximum tensile strength and elongation (121.2 MPa and 4.3%) being achieved at the heat input of 44 J/mm. The joint fracture occurs in heat-affected zone,and the fracture surface exhibits brittle fracture features.