Transmission is an important optical parameter in TFT-LCD products. In order to reach some optical requirements, such as view angle, the transmission axis on polarizer film of TFT-LCD product is changed. However, there are obvious differences in the transmission of the TFT-LCD products which have the same bare cell design but different transmission axis on polarizer films. The polarizer film and the backlight unit should be matched. In this paper, the experimental and theoretical studies show that the polarization property of backlight unit is the primary factor on the obvious differences in the transmission. The polarization property is mostly influenced by the prism film. The main polarization direction is approximately perpendicular to the prism direction. In the condition of 3° prism backlight unit structure, the transmission difference and the degree of polarization of backlight unit are linear relations. The linearity is more than 0.99.