A transmission device of a window shade includes a top box, a bottom bar and a shade located between the top box and the bottom bar. A first scrolling member and a second scrolling member are respectively connected to the first and second rails in the top box, and a movable unit is movably engaged with the first rail and between the first and second scrolling members. A transmission cord wraps around the first and second roller units of the movable unit, and the transmission cord wraps around the first and second roller units of the first scrolling member. Two ends of the transmission cord extend through the top box, the shade and are connected to the bottom bar. The length of the transmission cord can be stored between the first and second rollers so as to be cooperated with a longer shade.