A cordless curtain assembly includes a head box, a control unit, a fixed part, a movable part, a spring, an operation cord and a bottom bar. The curtain body is connected between the head box and the bottom bar. The fixed part is fixed to the head box and the movable part is located between the fixed part and the spring which is connected between the movable part and a fixed member of the head box. The user pulls or pushes the bottom bar move the movable part and the spring so as to operate the operation cord to control the position of the curtain body.