Birefringence and polarization dispersion caused by: 1) elliptical core, 2) twist, 3) pure bending, 4) transverse pressure, and 5) axial tension are studied by treating these deformations as perturbations to step-index single-mode fiber with a round core. These effects ate formulated in terms of fiber structure and perturbation parameters and are compared comprehensively. Birefringence depends strongly on factors 1)-4), while polarization dispersion depends on factors 1), 3), and 4). Polarization performance of twisted elliptical core fibers was also studied. Twist in an elliptical core fiber gives rise to a polarization whose major axis points in an intrinsic direction with an azimuthal angle , where φtis the torsion per unit length and is the fiber length.