The density and crystallinity of polypropylene fiber were measured on the moving threadline during the melt spinning process. Heretofore, threadline densities have never been measured on-line. These density measurements were accomplished by taking parallel, on-line measurements of fiber diameter, fiber velocity, polymer mass flow rate, fiber temperature, and fiber birefringence. Under certain spinning conditions, a distinct rise in density occurs along the threadline. This rise in density corresponds well with the rise in crystallinity as measured by birefringence.