In this paper, we propose novel measurement method that can measure the alignment states of the object in four degrees of freedom, simultaneously; those are pitch, roll, yaw, and height variation. This method mainly utilizes a laser diode, a reflective-type diffraction grating, and two-dimensional position-sensing detectors. The diffraction grating is attached on the object that we are interested. The pitch, roll, yaw angle and height variation can be obtained from the geometric relation among the distance between the grating surface and sensors, displacement of beam spot on sensor surfaces, and angle of diffraction. The experiments to verify the feasibility of the method are carried out. The repeatability and comparison of the measured values and numerical values are verified by experiments. The experimental results show good matches with theoretical values and good repeatability. The results show that our proposed method is effective for measuring 4 DOF of object.