The nep number in yarn depends on both the nep number in the raw material and changes in the number and size of neps in the spinning process. Therefore, the two problems need to be analyzed separately. The first problem consists of testing the value changes of some cotton parameters on different processing stages, which influence the appearance and quality of rotor yarns made from different raw materials. The second problem is an analysis of the reasons for mechanical nep creation in raw materials and during processing. In order to explain the phenomenon of mechanical nep formation, which in our opinion is related to fiber buckling, we derive the theoretical relationship between the cotton fiber maturity parameter, called the coefficient of circularity, and the minimal inertia radius of a fiber cross section, assuming both ellipse and flattened ring shapes of fiber cross sections. The correlation coefficients between the mechanical nep number and the other cotton fiber parameters are determined. The minimal inertia radius and maturity ratio are significantly correlated with the mechanical nep number in raw material. We can draw a hypothesis that the minimal inertia radius and fiber slenderness are the most relevant parameters for mechanical nep formation during processing.