Background: Injectable poly-L-lactic acid (PLLA) carries the risk of formation of nodules, papules, or lumps. Various methods including sonication have been employed to eliminate these adverse effects of PLLA. Objectives: This study investigated the change in size, distribution and properties of PLLA particles after sonication, and the duration of sonication needed to reach the ideal particle size. Methods: Two indicators, the average size of PLLA particles and diameter at 90%, were measured at each time point. The change of standard deviation of PLLA particle size over time and the crystalline remnants with 2 hours hydration only or 2 hours sonication were also measured to check the distribution. Results: The average particle size and diameter at 90% decreased drastically up to 10 minutes of sonication, and then increased slightly at 1 hour. Past 1 hour, the average size and diameter at 90% gradually decreased over time and reached 42.2μm and 75.7μm respectively at 2 hours. After 4 hours of sonication, the average size of particles was 34 μm, smaller than the smallest proper size required. Standard deviation decreased gradually over time, which means that more even distribution was obtained. In the particle periphery, PLLA microcrystals were less compact and more loosely distributed in the 2 hours sonication group. Conclusion: 2 hours sonication helps reduce the average size of PLLA particles and achieve more even distribution. Therefore, sonication may attribute to the safer use of PLLA.