We report a novel method that utilizes two lowest-order modes, HE(11) and TE(01), to probe birefringence induced in a bent circular-core optical fiber. The interference patterns of the two modes are examined by using a polarizer at the fiber output end. The contrast that is extracted from the interference pattern is periodically modulated as a function of phase retardation between the two modes, while the shift of the contrast function shows a strong dependence on the fiber birefringence as the polarizer rotates. Theoretical description followed by the test results demonstrates the feasibility of the method.