Density and melt index are two key properties in grading commercial polyolefin polymers. For quality assurance, these properties must be controlled as accurately as possible in the production plant. However, the lack of suitable in-line sensors for these properties makes feedback quality control difficult. In this study, an in-line density sensor using near infrared (NIR) spectroscopy is developed. The NIR spectra of molten polyethylene in flow are collected by a fiber-optic device attached to a single screw extruder. By the ratio of the absorption intensity at 1170 nn to that at 1213 nm, the densities of 14 grades of polyethylene were successfully measured. The results were very promising for quality control in the polyethylene production process.