Some demonstrative embodiments of the invention include a system and a method for disinfection of a liquid including monitoring the disinfection process. The system may include a conduit to carry flowing liquid to be disinfected, wherein the conduit comprises an inlet to receive the liquid, an outlet to discharge the liquid and walls transparent to ultraviolet radiations; an illumination source located within a transparent sleeve, wherein the transparent sleeve is immersed in the flowing liquid and the illumination source is to disinfect the liquid when passing through the conduit, a first light detector located externally to the conduit to detect light emitted by the illumination source and a second light detector located externally to the conduit to detect light emitted by the illumination source.