A novel deterministic sub-aperture magnetically assisted polishing method,Magnetorheological Finishing (MRF),was explored and some fundamental principles and key problems in MRF process were researched.The KDMRF-1000F MRF polishing machine developed by our group was introduced and two methods to establish the material removing model in polishing process were given.Then,how to realize a path planning based on dewell time was described.By using the KDMRF-1000F MRF polishing machine and KDMRW-1 water based MR fluid,one K4 flat work piece (80 mm in diameter) and one K9 sphere work piece (145 mm in diameter) were polished,experiments show that work piece one has a peak-to-valley (PV) value of 55.3 nm and a root-mean-square (RMS) value of 5.5 nm;and work piece two has a PV value of 40.5 nm and a RMS of 5 nm,which indicates that the surface roughnesses of two work pieces have been improved obviously.It is concluded that the MRF has the abilities to produce optical surfaces with high form accuracy,surface roughnesses and high efficiencies.