A window blind includes a retractable shading unit, a headrail connected with an upper end of the shading unit, a bottom rail connected with a lower end of the shading unit, at least one connecting cord extended through the shading unit and having a first end secured on the headrail and a second end secured on the bottom rail, a plurality of limit members mounted in the bottom rail, and a control unit mounted on the bottom rail. The second end of the connecting cord is provided with an elastic section having a length that is changed elastically to correspond to an expansion length of the shading unit. Thus, the elastic section of the connecting cord is extended or loosened when the shading unit is expanded or retracted to prevent from producing noise during movement of the shading unit.