The effects of isothermal aging at temperatures between 550-850 ?C for durations of 1-100 h on microstructure, tensile properties, and impact fracture behavior of 316L austenitic stainless steel weld metal have been investigated. For this purpose, various techniques including metallographic observations, measurement of ferrite content using ferritscope, and fractographic investigations have been utilized. It was found that increasing aging temperature and time lead to dissolution of delta ferrite and formation of a continuous network of sigma phase, followed by partial spherodization of sigma phase. These microstructural changes are found to induce a transition from ductile to brittle fracture mode which is accompanied by a sharp decrease in fracture strain and charpy impact energy, of up to 90%, and is characterized by specific types of fragmented and brittle microvoid fracture surface morphologies. The consequences of such behavior in regards to the selection of proper stress relief heat treatment temperature are discussed based on dividing the aging processing window into three distinct regions.