A sliding door structure for a vehicle includes a door configured to open and close an opening of a vehicle body, a door arm that connects the door to the vehicle body, a slider connected to an end of the door arm and slide-moved along a linear rail installed on the vehicle body, a hook disposed on the slider and rotated by the movement of the slider to rotate the door arm, a rotation pin disposed on the linear rail and rotating the hook by being coupled to or uncoupled from the hook when the slider is moved, a release button disposed on the slider and rotated in contact with the rotation pin when the slider is moved, and a gear lock in contact with the release button and rotated by the rotation of the release button to fix or release the rotation of the door arm.