A controller assembly for a window blind includes a drive mechanism and at least one transmission mechanism with a gear unit and a transmission unit. When the gear unit is driven by the drive mechanism to operate, first and second major gears thereof rotate in two opposite rotational directions. The transmission unit is selectively coupled to rotate with a selected one of the first and second major gears. When the transmission unit is coupled to rotate with one of the first and second major gears, the window blind is driven to wind up. When the transmission unit is coupled to rotate with the other one of the first and second major gears, the window blind is driven to wind down.