Background & Objectives: The heterotrophic plate count (HPC) bacteria and total coliforms are often used as the indicators of disinfection process performance. The aim of this study was to determine HPC indicator in drinking water distribution system in Ardebil city, Iran in 2012. Methods: In this descriptive-analytical study, 42 water samples were taken randomly from Ardebil drinking water distribution system and their HPC, free chlorine residual, turbidity, pH, and temperature were measured. While the pour-plate technique was used for the HPC test and the R2A culture media were used. Data analysis was descriptive statistical and regression test. Results: In 71.4% of the samples heterotrophic bacteria were present. In 9.6% of samples the HPC was higher than 500 CFU/ml. The mean of HPC indicator in water samples was 140.02 CFU/ml (SD=227.22) and the mean of free chlorine residual concentration was 0.33mg/l (SD=0.32). In 21.4% of samples the free chlorine residual concentration was less than 0.2mg/l and in 19% of samples was zero. The range of turbidity in samples was within 0.05-3NTU. There was a significant correlation between the HPC and turbidity (R=0.67), the HPC and pH (R=0.261), the HPC and free chlorine residual (R=-0.347), the HPC and temperature (R=-0.231). Conclusion: Monitoring of HPC in drinking water distribution system in Ardebil city at periodically in order to identification of contaminated areas and free chlorine residual concentration supply the range of 0.2-0.8 mg/l is necessary as an important variable and effective in the control of drinking water bacteriological quality.