Statement of Problem: Due to the significant correlation between salivary AST activity and periodontal disease, AST activity can be used as a useful marker for monitoring of the periodontal disease.Purpose: The aim of this study was to determine whether the initial phase of periodontal treatment could modify the salivary AST level in patients with moderate to severe periodontitis referred to Periodontics Department of Islamic Azad University, Dental school Tehran (1386).Materials and Method: In this clinical trial study (before & after), 33 patients (mean age of 45±9.75) with moderate to severe periodontitis were selected. Plaque index, pocket depth, clinical attachment level, bleeding on probing and salivary AST were determined before and two months after the initial phase of periodontal treatment for all patients. The effects of treatment were assessed by Paired-T and Wilcoxon Sign tests.Result: Following phase I periodontal treatment, plaque index, pocket depth, clinical attachment level and bleeding on probing decreased significantly ( p <0.05). Salivary AST changed from 40.3±15.5 to 32±12.1 ( p <0.01).Conclusion: Based on the statistically significant decrease in the salivary AST level after the initial phase of periodontal treatment, this biochemical marker may be used for evaluation of periodontal treatment.Key words: Periodontal disease, Scaling and rootplaning, Aspartate aminotransferase