We have developed the first and the smallest blood flow sensor composed of integrated circuits (ICs) fused with an optical microelectromechanical systems (MEMS) chip using system-in-package (SiP) technologies for application in a healthcare monitoring system. The probe of this blood flow sensor consists of three layers, and the optical MEMS chip is stacked as the top layer. Through silicon via (TSV), vertical-cavity surface-emitting laser (VCSEL) and cavities enable wafer-level packaging of the optical MEMS chip. The other two layers consisting of ICs are highly densified by SiP technology, and the volume of the probe is miniaturized to about one-sixth of our previously reported integrated laser Doppler blood flowmeter, an MEMS blood flow sensor to which SiP technology was not applied. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.