Combine harvester is one of the most needed agricultural machinery by farmers as it is increasingly difficult to find harvest worker. To develop the machine, Department of Agriculture and Biosystem Engineering collaborated with AGM Systems & Engineering to test the DSF75GT type combine harvester machine. The preliminary test shows that the machine performance is not optimal, especially when it is viewed from the grain damage percentage during the harvesting process. Based on the process, the harvesting principle of the machine includes the stages of cutting, feeding, threshing, and moving the grain from the threshing unit to the grain tank, and finally with unloading stage. Process to grain transfer with screw conveyor system is suspected to be the cause of grain damage. For this reason, this study aims to determine the machine parts which potentially contribute to the grain damage. The test was carried out in laboratory with two sample, i.e. harvested grain samples (GKP) and milled dry grain (GKG). The sample was inserted in to the threshing chamber which is then transferred by screw conveyor to the grain tank before being removed. Observation of the grain damage is carried out at 5 points on the screw conveyors. The results showed that there were 3 screws which contribute significantly to grain damage. From the result, the next step focuses on modification of the parts.