Subgingival irrigation with antimicrobial agent can be used as an adjunct to scaling and root planing (SRP). It has been demonstrated that the Streblus asper leaf extract has an antimicrobial effect. Objectives: this study aimed to determine the effect of subgingival irrigation of 80 mg/ml Streblus asper leaf extract solution as an adjunct to SRP on clinical and microbiological parameters in chronic periodontitis. Methods: The experimental design was the double-blinded placebo-controlled randomized clinical trial. A total of 42 subjects received oral hygiene instruction and SRP 4 weeks prior to study initiation. At the baseline, test group received SRP plus 80 mg/ml Streblus asper leaf extract solution and the control group received SRP plus normal saline irrigation. Both groups then were irrigated at week 1, 2, 3 and 4. Clinical parameters including Plaque Index (PI), percentage of plaque, Gingival Index (GI), Gingival Bleeding Index (GBI), probing depth (PD) and relative attachment level (RAL) were measured pre-treatment at week 0 and post-treatment at week 6 and 12. Microbiological parameters were measured pre-irrigation at weeks 0, 1, 2, 3, 4, 6 and 12. Results: all parameters except PI were significantly improved after the treatment in both groups. The test group had significant improvement in GI, GBI (p<0.001), percentage of plaque at week 6 (p=0.016), %site with reduced PI between week 6 and 12 (p=0.024), %site with P. gingivalis and/or A. actinomycetemcomitans at week 1 (p=0.005) and %site with P. gingivalis and/or A. actinomycetemcomitans reduced 10 times between week 0 and 12 (p=0.047). Conclusions: the use of 80 mg/ml Streblus asper leaf extract solution for irrigation as an adjunct to SRP is effective in the control of gingival inflammation, plaque formation and number of periodontopathic bacteria, when compared to the normal saline. This study was supported by a grant from Khon Kaen University