This paper reports the experimental research on micro-visualization of the growth process of cluster formation of ferromagnetic nano-particles in a water-based magnetic fluid on micro capillary flow. The purpose of this research is to observe the characteristics of primary clusters in micro capillary flow. The thermal behaviour of ferromagnetic nano-particles in micro capillary flow was observed through micro-visualization using the optical dark-field microscope system and particle tracking velocimetry (PTV) data processing system. Real-time visualization of primary clusters in a water-based magnetic fluid on micro capillary flow was carried out. Furthermore, the effect of magnetic field on the mixing process of using a water-based magnetic fluid in micro capillary flow was investigated.