As part of a system to optimize the cotton ginning process, a custom-built mass flow sensor was evaluated at USDA ARS Cotton Ginning Research Unit at Stoneville, Mississippi. The mass flow sensor was fabricated based on the principle of the sensor patented by Thomasson and Sui. The optical and electronic components of the sensor were housed in a single aluminum unit with mounting magnet, which made it easy to install and maintain. To obtain a calibration of the sensor, the total mass flow past the sensor was measured over known periods of time. To evaluate the effect of cotton cultivar on the sensor, a test of the sensor with two cotton cultivars was conducted using a micro-gin to compare lint mass flow with sensor output. Results showed that the sensor output was closely correlated with the lint mass, which passed through the sensor (r2 = 0.87), and the effect of cotton cultivar on the output of sensor was not significant. This demonstrated that the mass flow sensor can measure the total lint flow over a period of time in the gin and provide valuable information to the ginner.