The laser-diffraction technique has been applied to design a slit rheometer for measuring red blood cell deformability over a range of shear stress. Flow-rate and pressure-drop measurements are replaced with a measurement of pressure variation with time. Using a precision pressure transducer, one can measure the variation of pressure in the vacuum chamber, P(t), from which the stress and shear rate are mathematically calculated. In addition, a laser beam traverses a diluted blood suspension and is diffracted by RBCs in the volume. The diffraction patterns are captured by a CCD-video camera, linked to a frame grabber integrated with a computer. When deforming under decreasing shear stress, RBCs change gradually from the prolate ellipsoid towards a circular biconcave morphology. The Elongation Index(EI) as a measure of RBC deformability is determined from an isointensity curve in the diffraction pattern sing an ellipse-fitting program. The advantages of the design are simplicity, i.e., ease of operation and no moving parts, low cost, short operating time, and the disposable kit which is contacted with blood sample.