Abstarct: In the separation process by applying magnetic force, the particles can be separated and diverted depending on the magnetic susceptibility and size. In this paper, a numerical solution was presented to analyze and control the motion of particles in a rectangular microchannel to obtain particle trajectories due to the effect of magnetic field and fluid flow. By applying a permanent magnet, two-dimensional and time-dependent movement of non-magnetic particles in a ferrofluid flow inside the T-shape microchannel is investigated. The distribution of the magnetic force on the particles and their trajectories were first confirmed by analytical and empirical results, and then three models were presented to increase the separation efficiency by optimizing the magnetic field. To compare the separation efficiency, the models with different geometries with a nickel laxyer (in the third model) were proposed. Non-magnetic particles of polystyrene with the diameter of 0.2 to 7 μm and EMG 408 as the ferrofluid have been used in this method. The upper area of the central line of the microchannel is considered as the outlet 1 and the area below the central line as outlet 2. The results show that the separation efficiency in the third model is more than the first and second models. For each of the proposed models, two threshold values of particle size were reported; so that all particles with the size greater than the first threshold value are exited only from the first outlet and all particles with a smaller size than the second threshold value are only exited from the second outlet. The threshold values for the first and second outlets were obtained respectively 7 and 0.35 μm in the first model, 6 and 0.3 μm in the second model and 4.5 and 0.2 μm in the third model.