Molecular orientation of low-mass compounds (LMCs) in a cross-linked rubber is studied in order to obtain basic information on the material design of an optical anisotropic film. A small amount of LMCs such as 4-cyano-4'-pentylbiphenyl (5CB), phosphoric acid tricresyl ester (TCP), and styrene-based tackifier (TF) is added into polybutadiene rubber (BR) in an internal batch mixer at room temperature with a cross-linking agent. After cross-linking reaction in a compression-molding machine, the sheet samples are used to evaluate the orientation birefringence during stretching. The rectangular films, cut out from the cross-linked sheets, are set in a uniaxial stretching machine equipped with an optical system to measure both the retardation and the stress-strain curve simultaneously. It is confirmed that orientation birefringence is proportional to the stress for not only pure cross-linked BR, and also cross-linked BR containing LMCs. The stress-optical coefficient CR for pure BR is calculated to be 3.2 GPa-1 at 633 nm, which is almost identical to the previous data. Furthermore, CR is enhanced by the addition of 5CB and TCP, whereas the TF addition decreases the birefringence.