A head mounted display apparatus and a calibrating method thereof are provided. The HMD apparatus has a dual camera unit. The method includes: disposing a reference object in front of the dual camera unit, wherein the reference object has at least one physical feature; capturing a plurality of images of the reference object by the dual camera unit, and generating a distance information of the reference object according to the images; determining whether the dual camera unit being deformed or not by analyzing the distance information; and calibrating the HMD apparatus if the dual camera unit being deformed.