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Stress-induced birefringence of side-tunnel type polarization-maintaining optical fibers is presented. After checking the accuracy of finite-element calculations, the thermal stress analyses of various side-tunnel fibers are performed and the relation between the fiber parameters and the stress-induced birefringence is systematically investigated compared with the geometrical birefringence. As a result, it is found that for some cross-sectional shape, the stress-induced birefringence is more dominant than the geometrical one even in the side-tunnel fibers. We also investigate in detail the axial refractive-index and the axial stress of the fiber and compare them with those of ordinary step-index circular fibers.