Our previous study shows that poly phenylene sulfide (PPS) provides relatively high mechanical quality factors (Q factors) at ultrasonic frequency compared to other commonly-used functional polymers, and has potential as the vibrating body of a functional ultrasonic transducer. Since PPS has semicrystalline frameworks, its Q factors may be enhanced through thermal annealing. Prior to examining this feasibility, we developed a method for estimating Q factors under high-amplitude longitudinal vibration. Q factor is originally defined as the ratio of the reactive energy to the dissipated energy, both of which are calculated from the vibration velocities on the outer surface of a cylindrical specimen. Using this method, we experimentally investigated how annealing temperatures and times affect Q factors of PPS. The results demonstrate that thermal annealing is an effective way to enhance Q factors of PPS at the heating temperatures of 100 °C and 150 °C, relatively high compared to the glass transition temperature of PPS (90–95 °C). The Q factors at several tens of kilohertz are almost doubled after annealing owing to the enhancements in degrees of crystallinity. As the annealing time increases, the Q factors initially become higher, and gradually approach their saturated values at a sufficiently long time. Besides, annealing temperatures affect the change rates in Q factors, but have no observable effect on the saturated Q factors.