A method for evaluating red blood cell (RBC) deformability by different channel width in a microfluidic device is proposed. While conventional methods usually have only one test channel to deform RBCs for the evaluation, a design including three test channels with different width is utilized in this work. The proposed design have the advantage of generating a wider range of deformation to each RBC, thus the RBC deformability can be appropriately determined. Experiments on normal RBCs are conducted, and RBC motion through the channels is recorded and analyzed. The velocity drop, the velocity difference between a RBC and flowing fluid, is utilized as an index for the RBC response during deformation. According to the analysis, the relation between the velocity drop and amount of deformation of the RBCs is found nicely fitted with a modified contact model with R2 = 0.91, and the RBC deformability is successfully evaluated as in two deformability constants Cd and ζ.