A system for evaluating deformability of red blood cells (RBCs) from whole blood is proposed and tested. From previous studies, it is known that RBC characteristics are significantly changed under different liquids, such as plasma, saline, PBS or MAP. Since the plasma of blood is the most natural and friendly environment for RBCs, it would be the best if RBCs can be evaluated in plasma. For this purpose, we developed a new on chip system for evaluating whole blood directly. The plasma is firstly separate from whole blood by a sub stream, and later join the main stream again dilute RBC for achieving single cell evaluation. The deformability index based on cell size change is utilized for the evaluation. The pressure transmitter is used for manipulating RBCs with avoiding mixing between plasma and actuating fluid. From the experiment, we successfully evaluate RBCs without using any artificial buffer.