This study proposes an actively controllable method for fabrication of polymer microlens arrays. Hexagonal arrays of ferrofluid droplets, which were utilized as the mother-mold masters for microlens molding, were generated by inducing the magnetic hydrodynamic instability under different magnetic fields. To fabricate numerous cavity molds for microlens array molding, we used the field-dependent shapes of the ferrofluid droplets to create different sized molds. For the fabricated microlens arrays, their field-dependent bottom areas, lens height, radii of curvature, and the fill-factor were discussed. Furthermore, performances of the microlenses, including the numerical aperture, focal length, and depth of field, were tested.Index Terms-Ferrofluid droplet, magnetic hydrodynamic instability, microlens array.