Abstract: The present invention relates to an apparatus and method for real-time automatically taking the length, width and height of a rectangular object that is moved on a conveyor belt. The method of taking the dimensions of a 3D object, the method comprising the steps of: a) obtaining an object image having the 3D object; b) detecting all edges within a region of interest of the 3D object; c) extracting line segments from the edges of the 3D object and then extracting features of the 3D object from the line segments; and d) generating 3D models based on the features of the 3D object and taking the dimensions of the 3D object from the 3D models.