This paper introduces the small force measurement system at micronewton level at Center for Measurement Standards, Industrial Technology Research Institute (CMS/ITRI). The construction of the system started in 2009 and completed in 2014. The force measurement system consists of three main parts: a monolithic flexure stage, a three planar electrodes capacitor and a digital controller. The three planar electrodes capacitor is utilized simultaneously as a position sensor and an electrostatic force generator. Accurate electrostatic force is generated by constantly adjusting and maintaining gap distances between three planar electrodes. The lowest force uncertainty was evaluated to be 4.7 nN. The previous results show that the relative standard uncertainty at $10\ \mu \mathrm{N}$ and $100\ \mu \mathrm{N}$ are $2.3\times 10^{-4}$ and $7.4\times 10^{-5}$ , respectively. At current stage, the mass of mg level realized by this system has a slightly larger uncertainty than that obtained from traditional subdivision method. We discuss future works to further improve this system.
