Introduction: Bacteria and their by-products are the main cause of pulpal and periapical diseases. Complete debridment and disinfection of the root canal is essential in order to sustain the long term successful results following root canal therapy. This study aims at comparing the antibacterial effect of NaoCl, Thyme oil, Hydrated Calcium Oxide and Normal Saline on selected bacteria isolated from infected root canals.Materials and Methods: Bacteria were collected from 25 infected root canals of patients who had not received antibiotics. The bacteria were collected in the tubes and transfered for bacteriologic examination. Streptococcous viridance and black Pigmented Bacteroid groups were detected by specific bacteriological examination. The experimental solutions were serially diluted 8-folds in saline. A certain volume of bacteria was added to each diluted solution. After1, 5 and 15 minutes, the solution of each tube was poled onto 3% sheep blood agar plates and incubated at 37oc for 48 hours. The grown colonies were counted and the results were recorded.Results: The results of this study showed a significant antibacterial effect of thyme oil, hydrated calcium hydroxide and sodium hypocholorite on selected bacteria even in low concentration. Normal saline did not show any antibacterial effect.Conclusion: These findings recommended further studies on other aspects such as tissue toxicity, tissue solving and biocompatibility properties of these solutions to be applied for clinical use.Key words: Thyme oil, Hydrated Calcium Oxide, Irrigating solution