This paper presents a novel simple Y-type micromixer based on stable water suspensions of magnetic nanoparticles (i.e. ferrofluids). An electromagnet driven by an AC power source is used to induce transient interactive flows between a ferrofluid and DI water. The alternative magnetic field causes the ferrofluid to expand significantly and uniformly toward DI water associated with a great number of extremely fine fingering structures on the interface in the microchannel. Different magnetic strengths of the electromagnet were applied by adjusting the magnitude of AC supplied power at frequency of 45 Hz. The results show, due to the magnetic fields, two fluids mix with each other efficiently (mixing ratio can be as high as 95%). When the magnetic field is high enough, the labyrinthine fingering instability take place. This phenomenon is favorable for the fluid mixing. In addition, the increasing magnetic field enhances the efficiency of the mixing apparently.