Particle separation based on microfluidic technology offers a simple, reliable, and low-cost approach for the diagnosis of diseases. The separation concept can be extended to genetic engineering, cell transplantation, and immunology. This paper reports a simple microfluidic platform for the separation of diamagnetic particles of different sizes utilizing parallel ferrofluid streams. The ferrofluid streams with predefined concentrations of magnetic nanoparticles promote negative magnetophoresis and are able to separate a particle mixture with a subtle size variation. Numerical simulation was used to optimise the magnetic field gradient, e.g. the number and position of the external permanent magnets. The effect of flow rate ratio and the concentration distribution were analyzed by the simulation and validated by experiments. Furthermore, two-stream and three-stream ferrofluid configurations were evaluated to find the optimum separation performance. The experimental results show a maximum separation efficiency of 78% and 75% with three-stream configuration for 3.2-μm and 4.8-μm particles, respectively.