Disinfection ability of UVC diodes (wavelengths 275 nm, irradiation flux 45 mW) manufactured by UV:ON (Republic of Korea) was investigated on a laboratory strain of E.coli. Tests included irradiation of bacteria by 1–6 diodes simultaneously during 10–360 s. Irradiation was done in two variants of tests: on agar surface or in model water (0.5–1.0 cm of water layer). The initial bacteria concentrations were 32,000–163,000 CFU/ml. At the same values of irradiation dosage (calculated as number of diodes multiplied by time of their work) more significant decrease of microbial concentration was at shorter time and bigger number of simultaneously working diodes. The radiation dose to achieve disinfection (absence of viable microbes) and 3-log inactivation (needed for drinking water treatment) were calculated using manufacturer’s information of irradiation flux. They appeared to be close in both variants of tests and were 81–161 mJ/cm2 for disinfection and 40–81 mJ/cm2 for 3-log inactivation. The volume dose for flow-through water disinfection unit was also calculated and compared with manufacturer’s information.