Objective Periodontitis is an inflammatory disease of tooth-supporting tissue, caused by the bacterium such as Porphyromonas gingivalis. Inflammation that not treated can lead to resorption of alveolar bone. It is suggested that supportive medicines are needed to regenerate bone, one of them is lemuru fish oil gel (Sardinella Longiceps). Lemuru fish oil can act as an immunomodulator and anti-bacterium. Flavonoid content in fish oil plays a role to inhibit lipid peroxidation so it is effective in killing bacteria that can reduce LPS production. The content of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in lemuru fish oil lead to modulate IL-10, inhibition the production of pro-inflammatory cytokines such as interleukin-1 beta (IL-1 β), prostaglandin-E2 (PGE2), tumor necrosis factor alfa (TNFα) that will decrease receptor activator of nuclear factor kappa-Β-ligand (RANK-RANKL) band and result is increased osteoblast, decreased osteoclasts number which speeds up the bone regeneration process Objective: To determine the effect of topically applying lemuru fish oil in on periodontal Wistar Rats after induction of Periodontitis with P.gingivalis bacteria. Materials and Methods Experimental research laboratories with post-test only control group design. Samples of 25 male Wistar rats divided into 5 groups. K-(no induction), K (induced with P. gingivalis), P1 (induced with P. gingivalis and 10% lemuru oil gel therapy), P2 (induced with P. gingivalis and 20% lemuru fish oil gel therapy), P3 (induced with P. gingivalis and 40% lemuru oil gel therapy). Induction of the bacterium P.gingivalis was carried out, then the lemuru fish oil gel was given to gingival sulcus for 14 days. Observations were made by calculating osteoblast and osteoclast cells in histology samples using HE (Hematoxylin Eosin). Results The LSD statistic test showed a significant difference between the P. Gingivalis induction group with no treatment and treatment group using lemuru fish oil. The treatment using lemuru fish oil gel can increase the number of osteoblasts and decrease the number of osteoclast to the induction of P.gingivalis bacteria, with the most effective number of osteoblasts found in P2 group with a concentration of 20% lemuru fish oil gel therapy. Conclusion Topical Lemuru Fish oil gel can be used as alveolar bone regeneration therapy on periodontitis induced with P.gingivalis.