A capacitance-based void fraction sensor has been developed for the rocket or airbreathing engines, which is simple and do not disturb the flow. Typical conventional sensors usually have two concave electrodes mounted on the outer wall of the dielectric tube. They are relatively low accuracy if they have a noise shield; the maximum measurement error is over 30% in our research. The aim of this study is to improve the measurement accuracy while keeping the advantage of simplicity, mountability and non-intrusive characteristics. A theoretical formulae and electromagnetic field analysis, EFA, are used to design the sensors and are compared to an experiment using air/silicon-oil mixture flow. As the result, a newly developed asymmetrical type sensor which consists of asymmetric flat electrodes with side walls shows good performance; the inaccuracy between true void fraction and measured void fraction is 6% for the stratified flow.