Magnetorheological finishing (MRF) is a novel precision optical machining technology. Owing to its flexible finishing process, MRF can eliminate subsurface damage, smooth rms micro roughness and correct surface figure errors. The finishing process can be easily controlled by a computer. Material removal model in MRF is established. According to Preston equation in optical machining, mathematic model of material removal rate in MRF rotating at fixed rate is established through hydrodynamic analysis of the MR fluid flow in the polishing zone. The validity of the model is examined by the experimental results.