Transmission and extinction characteristics of a novel double-layer sub wavelength metal grating polarizer at visible region are investigated theoretically. The difference between the polarizer proposed and conventional lies in that the high refractive index dielectric layer is inserted between the substrate and grating,which icrease the transmissivity and extinction ratios. By using rigorous couple-wave analysis (RCWA),the transmissivity and extinction ratios of transverse-magnetic wave (TM) polarization are analyzed as a function of the thickness of the high refractive index dielectric layer and the duty cycle. Simulation results show that the transmissivity is more uniform compared with that of the conventional type in visible region. When the incident angle range is 0-60°,the TM transmissvity is still about 79% and the extinction ratios is higher than 50 dB. The polarizers is very compact and suitable for backlight unit used in liquid crystal devices.