To efficiently operate microwave assisted nanoparticle synthesis, thermal behavior around aggregate during irradiation must be well-understood to ensure stable operation. The key information in this regard are aggregate size and shape. In this study, aggregate size before the irradiation and maximum bubble size during the irradiation were measured at the different induction time and varying number density of primary particle. It was found that maximum bubble size is more closely related to the number density of aggregate than the number density of primary particles. Further analysis on the results indicates that aggregate size and shape can be estimated using known data of bubble size. These findings provide useful information on how microwave assisted nanoparticle formation can be better operated. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.