A new method for preparing functional O/W microcapsules using a process involving O/W/O emulsion as particle formation was developed. Coenzyme Q10 (CoQ10) or reduced coenzyme Q10 (QH) was used as the core substance. QH oxidized fast when exposed to air. O/W microcapsules were manufactured by conventional liquid phase drying method (LPD). The purpose of this study is to develop a simple method of estimating drop diameter which is possible to evaluate immediately the mean drop diameter during the microencapsulation process without the usual photographic measurement. This developed estimation is possible to predict a Sauter mean diameter by measuring the amount of inner CoQ10 released from O/W emulsion droplet. The amount of inner oil phase released from O/W emulsion has correlation with increased total surface area of O/W emulsion droplet caused by breaking droplet. Released rate of CoQ10 from O/W emulsion droplet to outer continuous phase under different rotational speed and emulsion viscosity was measured with an absorption spectrometer. As a result of the changes of released inner CoQ10 amount, droplet breakage under low emulsion viscosity was promoted by agitation speed. It is concluded that droplet dispersion state during manufacturing of O/W microcapsules was evaluated well by applying the developed estimation method. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.