The density and crystallinity of polyester fiber were measured on the moving threadline during the melt spinning process. The density was calculated by applying the continuity equation at points along the length of the threadline. Experimental inputs to the equation included parallel, on-line measurements of fiber diameter, fiber velocity, polymer mass flowrate, fiber temperature, and fiber birefringence. When spinning speeds exceeded 4500 m/min, a distinct rise in density occurred along the threadline. This rise corresponded with the rise in birefringence.