The deionized water and w(Fe₃O₄) =0. 5% of magnetic nano-fluids were taken as the working fluids to perform saturated flow boiling transfer heat experiments in the three different sizes of rectangular micro-channels．And the effect on the characteristics of CHF by nano-fluids was considered mass flow rate and magnetic field in the process of the flow boiling transfer heat was explored． The experimental results shows that the CHF values of deioni-zed water and 0. 5% magnetic fluid both increase with the increased mass flow rate． And the increase of CHF value is relatively obvious when the mass flow rate is low． The CHF values of 0. 5% of magnetic nano-fluids can be in-creased by approximately 71% ～157% than deionized water when there is no magnetic field． The CHF values of 0. 5% of magnetic fluid increase with magnetic field strengthening and the increase rate is about 4% ～ 17. 4%．Compared the experimental values with the predicted values of Kosar model，the MAE values of deionized water and magnetic fluid are 25% and 30% respectively． After the modification of Kosar model，the MAE values are both less than 15%． Obviously，the prediction of the experimental results is improved．