Polarized polymer-based light-emitting diodes (PLEDs) have been fabricated by inserting a very thin photoaligned polyimide film into the device structure. The photoaligned polyimide film was used to form a highly oriented layer of light-emitting polymer, poly (9,9-dioctylfluorenyl-2,7-diyl) (PFO). The polyimide contains azobenzene in the backbone structure, allowing us to control the alignment of its backbone structure by optical treatment. Since the photoalignment treatment is scratch-free, the thickness of the photoaligned film can be reduced below 4 nm without decreasing its alignment ability for PFO. Even though polyimide is a good insulator, such a very thin photoaligned film can be inserted into PLED structures. We examined the influence of the thickness of the photoaligned polyimide film and the light-emitting layer on the polarization ratio and the current efficiency of the polarized PLEDs. Using a 2.9 nm-thick polyimide photoalignment layer and a 101 nm-thick PFO light-emitting layer, we have succeeded in fabricating a polarized PLED with a current efficiency of 0.3 cd/A at 150cd/m2 and polarization ratios of 46 at 434 nm, 27 at 460 nm, and 20 for integrated intensity from 400 to 600 nm.