A curtain wall system is assembled by a plurality of panels supported by a curtain-wall supporting mullion. The panels comprise one or more functional types of panels: transition panels and adjacent panels that can be joined at the top or/and bottom side of the transition panels because each of the panels has a male frame member is used to engage with the adjacent panels each having a female frame member. In each transition panel, a frame is assembled by a jamb frame member connecting with the upper frame member and the lower frame member. As a result, the curtain wall system can be easily erected in a bottom-up procedure, a top-down procedure, or the combination of the two.