As a deterministic magnetically assisted polishing method,magnetorheological finishing (MRF) has the ability to produce optical surfaces with high from accuracy,high efficiency and low sub-surface damage. The principle and method of MRF process are introduced and the key techniques required to implement a MRF process are also discussed. To demonstrate a MRF process,one K9 flat (80 mm in diameter) is polished with KDMRF-1000F polishing machine and KDMRW-1 water based magnetorheological (MR) fluids. Through one MRF iteration (4.39 min),its form accuracy peak-to-valley (PV) error is improved from initial 0.144 λ to final 0.06 λ (λ=632.8 nm),root-mean-square (RMS) is improved from initial 0.031 λ to final 0.01 λ,surface iterative convergence ratio is 2.81 and roughness RMS is 0.345 nm. The result shows that MRF is a deterministic process,which results in high form accuracy,surface roughness and high efficiency.