In this work, the effects of cold plasma treatment on the mechanical strength of polyethyleneterephthalate (PET) fibers has been verified. Single fibers were treated with oxygen and a mixture of oxygen and tetrafluoroethylene in a cold plasma reactor for 30, 100 and 200 s. The single fibers were then tested in tensile mode and the mechanical strength was analyzed by using the Weibull distribution function.