Design of glass curtain wall (CW) systems has long been under the influence of structural, thermal and daylighting performance requirements as well as cost and aesthetic concerns. However, environmental life-cycle impacts of the systems are usually ignored when selecting the suitable materials for mullions and glazing units. This paper intends to examine the effect of mullion material change on the environmental impacts of a typical CW system over its life-cycle. The mullion materials studied in this paper include extruded aluminum, carbon steel and glulam timber. The environmental impact categories of interest include global warming, acidification, eutrophication and human toxicity. In addition, the paper applies a process-based Attritional Life Cycle Assessment (LCA) technique to achieve its objectives. According to the results of this study, an extruded aluminum CW system contributes most to the environmental impact categories of interest in this research while a glulam timber CW makes the least contribution. Contribution of steel CW systems falls in-between.