Abstract: Objective To investigate hygienic status of dental clinic water in departments of stomatology in Guangzhou so as to provide scientific basis for control of nosocomial infection. Methods Samples of dental source water, dental mouth wash, washing water, and hand-piece water in self-purifying-typed and storage-typed dental unit waterlines of medical institutions at different levels were collected by multi-stage stratified random sampling method and general survey. Bacteria of water samples were isolated, identified and evaluated based on health standard for disinfection in hospitals (GB 15982-2012) and health standards for drinking water quality (GB 5749-2006). Total bacteria counts were expressed by the median (P25 and P75). Results A total of 204 water samples were collected from self-purifying-typed dental unit waterlines (68 samples for each of dental mouth wash, washing water, and hand-piece water) and the total bacteria counts were 40.00 (10.50-373.50) CFU/mL in mouth wash, 1 141.50 (185.00-2 995.00) CFU/mL in washing water, and 3 302.00 (1 397.75-5 601.25) CFU/mL in hand-piece water. There were statistically significant differences in total bacteria counts among different treatment water samples of self-purifying-typed dental unit waterlines (P <0.01). Meanwhile, 66 water samples were collected from storage-typed dental unit waterlines (22 samples for each of dental source water, washing water, and hand-piece water) and the total bacteria counts were 225.00 (17.50-1 619.75) CFU/mL in dental source water，2 160.00 (440.00-3 652.00) CFU/mL in washing water, and 2 665.50(252.00-5 636.00) CFU/mL in hand-piece water, respectively. Significant differences were found in total bacteria counts among different treatment water samples in storage-typed dental unit waterlines (P <0.05). Dominant bacteria were Sphingomonas paucimobilis, Cupriavidus pauculus, Burkholderia cepacia, and Ralstonia pickettii . Conclusion The dental unit waterlines were contaminated seriously in departments of stomatology in medical institutions of Guangzhou. Regular monitoring and disinfection measures should be strengthened to prevent and control nosocomial infection.