A measurement method and a measurement device are disclosed. The measurement method is adapted to a 3D display capable of producing a plurality of viewing zones, which includes following steps. At least a part of a display area of the 3D display displays an image of one of the viewing zones. Light radiation amounts of a first position of the 3D display are detected with a light detector from different viewing angles. A viewing angle with a local maximum light radiation amount is taken as a reference viewing angle, a distance between the light detector and the 3D display along the reference viewing angle is changed, and light radiation amounts of a plurality of different second positions of the 3D display are detected in different distances. A distance at which light radiation amounts of the second positions are most uniform is taken as the optimal viewing distance.