This article reports the scaling law of droplet formation of the paramagnetic ionic liquid (MIL) under magnetic field in a coaxial microfluidic device. Firstly, the effect of two phase flow rates and magnetic forces on the size of MIL droplet in a coaxial microfluidic device were studied systematically. Then, based on force analysis, a mathematical model for the prediction of the diameter of MIL droplets was developed. Furthermore, a novel method which can achieve in-situ formation and controlled coalescence of droplets was developed by using a designated magnetic field. (C) 2016 Elsevier Ltd. All rights reserved.