The purpose of this study is to investigate the behavior of red blood cells in order to pass through capillaries. In this research, the behavior of a single and multiple red blood cells are numerically investigated using a fluid-structure interaction (FSI) method. For calculations and comparisons of the deformation, displacements and stresses, four object points were selected and two different boundary conditions were applied to the problem. In the first, uniform velocity flow was applied at the inlet of the capillary and in the second model, the oscillating pressure was set. Through these investigations, the result reveals that the deformation in triple configuration of RBCs is more essential, sine the presence of the two additional cells above and beneath the original cell, will change the regime of flow and add extra amount to turbulency. By noticing this fact, in the uniform velocity flow pattern the results are expectable and convincing, because the displacement of triple array is greater than the single shape. But in the oscillating pressure pattern, there is no significant difference between the displacements of two corresponding points at two configurations. This output is in contrast with the results of uniform velocity flow and real situations. It shows that the assumption of uniform velocity flow at the beginning of the capillary is more logical than the oscillating pressure.