A computational method is developed to evaluate the relative amount of mesophase of polymer fibers from their two-dimensional (2D) wide-angle X-ray scattering (WAXS) patterns. This is achieved by first evaluating the percentage of oriented phases (POPs) from a 2D WAXS pattern. If there exists a crystalline phase, crystallinity is subtracted from the POPs to obtain the mesophase percentage. If no crystalline phase is observed from a WAXS pattern, the POPs is the mesophase percentage. To increase the computation accuracy, a center search algorithm is developed and a Fourier filter is adapted to perform data smoothing. The Bresenham midpoint circle algorithm is applied to accelerate the computation. This method is applied to compute the mesophase percentages of poly(trimethylene terephthalate) (PTT) fibers of take-up speeds ranging from 970 to 4185 m/min. Our results indicate that the mesophase percentage increases with take-up speeds up to 2550 m/min. With further increase in take-up speeds, the mesophase percentage begins to drop. This is because the mesophase is precursory to the crystalline phase and is depleted by the formation of the crystalline phase during spin-line stress-induced crystallization in the take-up speed range of 3050 to 4185 m/min. This finding is consistent with results on PET fibers.