Gray platelet syndrome (GPS) is one of primary hemostatic disorders with characteristics of moderate bleeding tendency, thrombocytopenia, gray platelet on Wright-Giemsa stained smear and absence of platelet -granule. It is known to be mostly inherited by autosomal dominance but not all. We report a case of gray platelet syndrome diagnosed in a woman with bleeding tendency such as easy bruise and evaluate clinical usefulness of mean platelet component (MPC), mean platelet volume (MPV) and platelet component distribution width (PCDW) using ADVIA 120 (Bayer Diagnostics, NY, USA).