A platelet is a discoid and anucleate microcytic cell of diameter 2-4μm. It contains various substances such as platelet-derived growth factor, β-thromboglobulin, and von Willbrand factor（vWF）in its α-granule, as well as ADP, ATP, and serotonin in its dense granule. Platelet membrane has some unique glycoprotein（GP）such as GP Ib/IX/V complex：the receptor for vWF, and GPVI：the receptor for collagen. These play an important role in body’s hemostatic mechanism. However, platelet is difficult to operate as an object of a clinical investigation, because it is easily-activated, makes form change, adheres, clumps, and eventually, it becomes micro particles（MP）after activations.Activated-platelet-derived MP has been produced from activated-platelet, which has strong procoagulant actions and important roles for a formation of thrombus after bleeding. On the other hand, MPs can also be produced by various cells such as monocytes, vascular endothelial cells, and cancers et al., and methods to identify sources of their MPs are not developed yet. We focus on developing highly-sensitive ELISA which enables to specifically measure aPLT-MP and engage in improving its diagnosis and prediction to patients with thrombosis. The progress of our research for various roles of platelets is reported here.