The use of metalworking fluids (MWFs) in the milling process can cause emissions of a large amount of oil particles, which can induce respiratory and immune system diseases of workers. This work examined the emission characteristics of oil particles caused by the milling process. A closed test chamber containing a real milling machine was built. The emission rates of oil particles in various sizes using five kinds of MWFs were measured, and the influence of the rotation speed of the spindle in the milling machine and the physical properties of MWFs on the oil particle emission characteristics was analysed. In addition, a prediction model of emission rate and size distribution of oil particles was introduced. Based on the measured data of oil particle emission rates in each size, the morphological particle size, distribution index and atomization coefficient in the prediction model are fitted. The deviation between the predicted and measured values of the emission rates is about 5%, which confirms the accuracy of the model. The prediction model is helpful for pollution control and ventilation strategy formulation in typical machining plants.