In conventional chair structure, the chair cushion is usually parallel with the floor, and the angle between the cushion and a backrest is usually less than 90 degrees, so the user may easily feel physically/physiologically uncomfortable, and the user may not sit on the chair for a long period of time. Furthermore, the user may suffer from pain or sickness if sitting on the chair on a regular basis. Therefore, there remains a need for a new and improved chair structure to overcome the problems stated above.