Loss in optical fiber coupling efficiency and transmission are computed for a telecommunication optical circulator. Optical performance degradation is due to thermally induced optical errors in the two beam splitter cubes. The computation of the optical errors is discussed for two materials and the effects illustrated. Bulk volumetric absorption of the incident laser radiation from the input optical fiber and surface absorption via the coatings on the beam splitter interface generate temperature gradients. Loss in optical fiber coupling efficiency is produced by wavefront error caused by thermal expansion effects, and refractive index changes with temperature and stress. Transmission loss in the optical circulator is caused by polarization errors generated by the effects of stress birefringence. The optical errors were computed using temperatures generated from a Thermal Desktop model and displacements and stresses generated by a MSC/Nastran finite element model. The optical errors were imposed upon a Code V optical model to compute loss in fiber coupling efficiency and transmission in the optical circulator.