A positioning apparatus of a cam plate for a vehicular air conditioning system may include the cam plate rotatably coupled to an air conditioning case by a cam shaft and including a guide slot, a plurality of positioning holes formed in the cam plate, a detent mechanism projecting from the air conditioning case toward the cam plate, and an arm rotatably coupled to the air conditioning case and including a connecting pin that may be slidably coupled to the guide slot of the cam plate, wherein, while the cam plate rotates about the cam shaft and the connecting pin included in the arm performs a camming action along the guide slot formed in the cam plate, a leading end of the detent mechanism may be selectively inserted into one of the positioning holes to detent the position of the cam plate when the connecting