A continuous variable valve duration system may include a camshaft, and a first cam portion including a first cam. The camshaft is inserted in the first cam portion. A relative phase angle of the first cam with respect to the camshaft is variable. The system may further include an inner bracket transmitting rotation of the cam shaft to the first cam portion, a slider housing in which the inner bracket is rotatably inserted and movable along a predetermined first direction, a rocker shaft, a first rocker arm rotatably disposed to the rocker shaft of which a first end contacts with the first cam and of which a second end is connected to a first valve, an upper housing connected with the slider housing and relatively movable with respect to the slider housing along a second direction vertical to the first direction and a control portion selectively moving the upper housing.