In the separation process of micro-particles by applying magnetic force, the particle can be separated and diverted depending on its magnetic susceptibility and size. In this paper, the motion of particles with different sizes were analyzed in a microchannel to obtain particle trajectories due to the effects of magnetic fields, Nickel Sputtering and fluid flow. By applying a permanent magnet, temporal movement of non-magnetic particles in a ferrofluid flow inside the T-shape microchannel was investigated. The distribution of the magnetic force on the particles and their trajectories were first confirmed by analytical and empirical data, and then three models (including magnet arrangements and Nickel layer) were presented to increase the separation efficiency by optimizing the magnetic field. Non-magnetic particles of polystyrene with the diameter of 0.2–7 μm and EMG 408 as the ferrofluid have been used in the model. The results show that the separation efficiency in the model with Nickel Sputtering is more than the others. For the proposed models, two threshold values of particle size were reported for the first time; So that all particles with the sizes greater than the first threshold value are completely sorted to the first outlet of the channel, meanwhile; the particles smaller than the second threshold value are only ejected from the second outlet. Therefore, a smart sorting can be accessible by the proposed technique. The particle diameter threshold values in the 1st, 2nd and 3rd models are obtained as 7, 6, 4.5 μm (i.e. first outlet) and 0.35, 0.3, 0.2 μm (i.e. second outlet), respectively. The threshold values were reported in the paper to design different tuning strategies.