A measurement system based on a W-type optical fibre sensor for determining the fat content in milk is investigated in this paper. The system consists of a light source, W-type optical fibre sensor, detector, amplifier, A/D converter, microprocessor and thermoelectric cooler (TEC). According to Mie-scattering theory, the standard models of the system at different temperatures (25°, 30°, 35°, 40° and 45 °C) are obtained. Evaluations of all the models at different temperatures are made, which illustrate that 40 °C is the optimal temperature for fat content in milk. At 40 °C, the linear relationship between the absorbance and the fat content is significant. Furthermore, a prediction experiment has been performed to confirm the validity of the standard model. As a result, the measurement system based on a W-type optical fibre sensor is capable of measuring the fat content in milk effectively and in real time.