Experimental studies on the mixing of granular materials in a screw feeder were conducted in this study. It was observed that the granular flow in a screw feeder is similar to plug flow. The degree of mixing of the screw feeder was found to be increased by either (1) increasing the shaft diameter, (2) decreasing the pitch, (3) increasing the flight diameter, (4) increasing the flight thickness, (5) increasing the angle between the upper feeder and the lower feeder, (6) increasing the length of the lower feeder, or (7) decreasing the rotational speeds of the upper screw and the lower screw. The degree of mixing was also increased by employing a screw with paddle and decreased by employing a tapering shaft screw, a cut-flight screw or a stepped-flight diameter screw. The degree of mixing was also found to be reduced if the ratio of rotational speeds of upper screw to lower screw was either too low or too high. The degree of mixing was found to increase whenever the feed order of a large granular material following a small one was inversed. The degree of mixing of the screw feeder fed with two different materials was higher than that with the same material.