In precision agriculture, farm characteristics data collected from various soil and yield monitoring sensors installed on agricultural machinery along with geographical data from GPS are combined to be transferred to GIS environment for site specific yield map development. Yield monitoring perhaps is the cornerstone of precision agriculture and for this purpose grain mass flow sensors make yield mapping possible. One way to determine grain mass flow is to measure the torque on the clean grain elevator shaft exerted from the power train chain on a combine harvester. Load cells may be used to obtain a proper indication of transferred torque from power train chain to the clean grain elevator shaft instantaneously taking into account on the go combine harvester vibrations. The load cell is placed on a sprocket on the pressure side of the power train chain. In this researchthe type of sensor explained above was used to measure grain mass flow. The sensor was calibrated with several varying mass flows. Data collected showed some noise due to vibration in the elevator chain and friction between the elevator paddles and the housing. Since the data was normally distributed, the averages were used for further processing. For evaluation purposes, sensor outputs for each wheat grain output rate from the elevator was recorded in a data logger within every 15 seconds time interval. Mean sensor's output variations curves unser fixed time intervals as related to grain elevator output was plotted which had and a coefficient corrolation of 0.99. Mean comparison test results showed that rate of elevator output have a significant effect on sensor's voltage output. As the rate of elevator output increased the sensor's voltage output increased as well. Keywords: Precision agriculture, mass flow sensor, clean grain elevator, combine harvester, yield map.