Miniaturization of electronics modules is always required for various medical applications including wearable technology, such as hearing aids, and implantable devices. Many types of high-density packaging technologies, such as package-on-package, bare-die stack, flex folded package and Through Si Via (TSV) technologies, have been proposed and used to fulfill the request. Among them, embedded die technology is one of the promising technologies to realize miniaturization and high-density packaging. We have developed WABE™ (wafer and board level device embedded) technology for embedding dies into multilayer flexible printed circuit (FPC) boards. The WABE package is comprised of thin dies (85 μm thickness), multi-layer polyimide, adhesive films and conductive paste. The dies are sandwiched by polyimide films with Cu circuits (FPCs). The conductive paste provides electrical connections between the layers as well as the layer and embedded die. First, each FPC layer is fabricated individually, and via holes are filled with conductive paste, and the dies are mounted on certain layers. Then, all layers undergo a one-step co-lamination process, and they are pressed to cure the adhesive material and conductive paste at the same time. This WABE technology has enabled multiple dies to be embedded by the one-step lamination process. Even if multiple dies are embedded, the footprint of a package can be reduced drastically by embedding multiple dies vertically in stacks. This paper describes the details of the results of fabricating a test vehicle with six embedded dies (three-dies in two stacks side-by-side). The fabricated test vehicle had 14 copper layers with less than 0.9 mm thickness. This paper also reports the results of various reliability testing on the package. These results were obtained by electrical measurements of daisy chain patterns formed between some of the layers. The fabricated test vehicle showed high reliability based on the results of a moisture and heat test and heat-shock test. These results show that the WABE technology to embed multiple dies vertically in polyimide film is one of the most promising packaging technologies to significantly miniaturize electronic circuits such as medical electronics.