Reflectance and transmittance are main optical property of optical thin films. Transmittance is easier to be measured and has a higher accuracy than reflectance. Therefore, it′s frequently used to fit the optical constant and thickness of the film. Many coatings work in the oblique incidence case in application, and its oblique transmittances Ts and Tp are needed. A perfect polarizer with high extinction ratio is needed in this case. However, it costs too much for the compact spectrophotometer to provide such a polarizer. In order to solve this problem, rotation-incidence-plane (RIP) method is investigated deeply and generalized. Because RIP method makes measurement with a partially polarized beam, and the output beam of the spectrophotometer is partially polarized, RIP method can measure the oblique transmittance under certain condition. Advanced investigation found that if the absolute value of the polarization factor of the measurement beam is bigger than 0.167 the RIP method can make an accurate measurement about the oblique transmittance of optical coatings, and the polarization factor bigger the accuracy higher. This method has a good accuracy in the wavelength region, in which the transmittance is nearly flat or not sensitive very much to the incidence angle, when it′s used to measure the Ts and Tp. What′s more, the method has a better accuracy when it′s used to measure the average transmittance.