Different molecular weight samples of a polyimide, 6FDA-PFMB, were synthesized from 2,2’-bis(3,4-dicarboxyphenyl) hexafluoropropane (6FDA) and 2,2’-bis(trifluoromethyl)-4,4’-diaminobiphenyl (PFMB) in refluxing m-cresol at high temperatures through a one-step polymerization route. Films of 6FDA-PFMB ranging from 1 to 100 mu m thickness were prepared via solution-casting and investigated using optical prism coupler wave-guide experiments. It has been found that these films exhibit a linear optical anisotropy in the refractive indices. Specifically, they have a large index along the in-plane direction and a smaller one in the out-of-plane direction. Films with this linear optical anisotropy have been recognized to possess uniaxial negative birefringence. For films with a fixed molecular weight, the refractive indices are constant in the him thickness below 15 mu m, and the birefringence gradually decreases with further increasing the film thickness. On the other hand, the refractive index along the out-of-plane direction decreases while the in-plane refractive index increases when the polyimide molecular weight increases. Such films can be used as negative birefringent compensators in twisted and super-twisted nematic liquid crystal displays to improve display viewing angles.