Strouhal number and base pressure coefficient of rectangular cylinders with a section of a width /height ratio of l∼9 were measured in a wind-tunnel and flow around the cylinders was visualized by .means of a smoke-wire technique. It has been found that there exists a new critical value of a width/height ratio, 6.0 where a value of Strouhal number changes with a discontinuity, accompanied with a drastic change of flow pattern. Flow characteristics in between two critical values of the width/height ratio of 2.8 and 6.0 were clarified ; a flow around a rectangular cylinder changes from a fully separated flow to a periodically reattached one at the ratio of 2.8, and over the ratio of 6.0, a stationarily reattached flow follows, accompanied with a separation bubble.