This paper describes observations of thrombus formation process on orifice flows by laser sheet. In the observation of thrombus formation process, the brightness of CCD image corresponds to protein or thrombus concentration. In this paper, thrombus formation rate was estimated by the gradient of brightness history near the wall. In addition, we changed flow rate using the same orifice type to compare the difference of the thrombus formation rate. As a result, the difference in thrombus formation process was seen by changing flow rate, and it was found that the threshold of the shear rate for the thrombus formation existed because of the platelet activation.