In grain yield monitoring system, the amount of clean grain mass flow rate to the storage bin is the most important yield property. In this research, an impact-plate type grain mass flow sensor was designed, developed and evaluated. After construction of the impact sensor, it was calibrated by loading the impact plate with static weights ranging from 0.5 to 4.5 kg every 0.5 kg and its linear response to the applied loads was proved with a correlation coefficient of 0.99. Then, grain mass flow measurement tests and data collection were conducted according to the ASABE standard S587, developed for grain mass flow sensors. The tests were conducted in three phases: 1- constant and steady state flow, 2- linear variation of flow, 3- oscillating flow. The results showed that the output of impact plate sensor varies proportionally and linearly with increasing wheat grain (Rowshan cultivar) mass flow rate. The error in prediction of actual flow rate was decreased by increasing the mass flow rate such that the calculated errors at 25%, 50%, 75% and 100% of flow capacity (4.25 kg s-1) were 8.3%, 6.3%, 5.2% and 4.9%, respectively. The high coefficient of determination (R2 = 0.9975) between accumulated mass flow data of impact plate sensor and the reference scale data indicated high accuracy and sensitivity of impact plate sensor in prediction of mass flow variations. The average percent error of impact sensor in variable flow rate in “ramp-up-ramp-down”, “ramp-down-ramp-up” and oscillating flows were 7.4%, 8.6% and 8.3%, respectively.