Polydioxanone Cog thread and poly-L-lactic acid (PLLA) thread have been used clinically for lifting and antiaging purposes. However, the histological changes in tissue and the efficacy of these 2 different types of thread have not been compared. This study used hematoxylin and eosin (H&E) staining, Masson's trichrome staining, and electron microscopy (EM) to compare the results associated with the use of Cog thread and PLLA thread in rat skin. Thirty female rats were divided into 3 groups of 10 rats each: a control group; a Cog group; and a PLLA group. Biopsy specimens obtained at 1, 3, and 6 months were examined using H&E, MT, and EM. Although significant increases were observed in dermal thickness and the numbers of fibroblasts in the PLLA group compared with the control group within the first month (p: .019), there were no significant differences between the Cog and control groups during this period (p: .245). Dermal thickness (p: .002) and numbers of fibroblasts (p: .001) were similar in samples obtained from the PLLA and Cog groups at 3 and 6 months, and both groups showed significantly improved outcomes compared with the control group. Poly-L-lactic acid and Cog sutures were effective in facial rejuvenation; both increased dermis thickness and stimulated collagen production.