Ultrasonic modification was used as a simply-operated and efficient method for improving the hydrophilicity and cytocompatibility of polyglycolic acid (PGA) and poly lactic-co-glycolic acid (PLGA, lactide:glycolic acid (LA:GA) = 10:90) fibers, and maintaining the tensile property at the same time. The fibers were pre-treated ultrasonically by dipping in the mixed solution composed of absolute ethyl alcohol and polyphosphoric acid (PPA) (volume ratio 1:1) at 250 W ultrasonic power for 6 min. Scanning electron microscopy was used to observe the surface morphology of PGA and PLGA fiber before and after modification. Fourier transform infrared spectroscopy was used to investigate the change of fiber chemical composition. X-ray diffraction and differential scanning calorimetry analysis showed that the crystalline degree of modified PGA and PLGA fibers decreased. The results of tensile test indicated that compared to that before modification, the breaking strength of modified PLGA increased, while the breaking strength of PGA fiber remained unchanged. The water contact angle of modified fiber was lower than that of unmodified fiber, showing higher hydrophilicity. The cell proliferation assay indicated that fibroblast cells attached and proliferated better on the modified fiber, which means the modified fibers possess good cytocompatibility. These results suggested that ultrasonic modification is an easy-operated and efficient method and the modified PGA and PLGA fibers could be useful in the biomedical textiles field.