Knowledge of differential mode delay data of polymer optical fiber plays an important role in studying the dispersion characteristic of the fiber. Differential mode delay of step index-polymer optical fiber (SI-POF) and gradient index-polymer optical fiber (GI-POF) with various lengths is measured by the frequency-domain phase-shift technique. Due to the fact that the mode coupling and the mode-dependent loss can be neglected for short fibers, the measured values of the differential mode delay for the 1 m fibers agree well with the theoretical ones. Therefore, the frequency-domain phase-shift technique provides a simple, convenient way to determine the differential mode delay of the polymer fibers with high accuracy. In addition, the mode coupling of POF can be analyzed to some extent by comparing the differential mode delay curves of the fibers with various lengths.