Abstract: An apparatus for continuous cold sterilization of a fluid which comprises at least one ultraviolet radiation source and at least one duct, through which the fluid flows, permeable to such radiation. The duct has a helically shaped portion around the ultraviolet radiation source arranged in a chamber having walls with reflective surfaces. The distance between the walls and the helical portion is sufficient to allow adequate circulation of air between them, whereby excessive heating of the fluid is avoided. In addition, at least the helical portion of the duct has an elliptically-shaped passage section with its major axis generally perpendicular to the direction of irradiation so as to reduce the thickness of fluid exposed to radiation and, thereby, increase treatment efficiency.