ObjectiveTo evaluate the difference in the configuration and quantity of biofilms in dental unit water lines (DUWL) before and after disinfected with sodium hypochlorite solution. MethodsThe samples of biofilms from DUWL supplied with 10 separate water sources were collected and the microorganisms were quantified after incubated for 48h, and the configuration of biofilms were observed with confocal laser scanning microscope (CLSM) and scanning electronic microscope (SEM). The sodium hypochlorite at concentration of 525 mg/L was used for disinfection of DUWL. ResultsThe number of bacteria in biofilms at the baseline was 1.7×103CFU/cm2 before disinfection and 0 CFU/cm2 after disinfection with sodium hypochlorite solution, there was significant difference between the two (t=12.03,P=0.02) . CLSM and SEM showed that before disinfection, biofilm existed in the inner wall of DUWL,and bacilli and cocci distributed in the matrix; after disinfection, the configuration of biofilms, particularly fencelike structures were destroyed, but matrix was still existed. ConclusionDUWL has been contaminated heavily with a number of microbes, and the inner walls of DUWL is covered with biofilm, which is composed of millions of bacteria. The high concentration of microbes in DUWL is bringing the high risk of cross infection to both dental professionals and patients. Sodium hypochlorite at the concentration of 525 mg/L may be used as a disinfection solution for DUWL with separate water source.