He aim of this work is to investigate the effect of diamond nanoparticles on blood microrheology, in particular, on the ability of red blood cells to deform in shear flow and to spontaneously aggregate. The research is motivated by the fact that the nanoparticles are supposed to be administered into a live organism via the blood flow intravenously. However, their possible effect on blood and its ability to normally flow along the vessels of different diameters is usually not accounted for. In order to study this effect by the methods of diffuse backward light scattering and laser diffractometry in vitro measurements were conducted of several parameters of aggregation kinetics and deformability index. Aqueous suspensions of nanoparticles with sizes from 5 to 500 nm were used, which were added into the samples of freshly drawn blood. Also, a theoretical analysis was performed of the effect of size distribution of the particles modeling red blood cells on the visibility of the diffraction pattern in the diffractometer. The shape of this pattern is used in ektacytometry to measure the deformability index of red blood cells. Besides, the interaction of diamond nanoparticles with blood plasma proteins albumin and γ-globulin in aqueous solutions also affecting the blood microrheology was studied with dynamic light scattering technique. It was shown that nanoparticles decrease the cell deformability in shear flow in in vitro experiments, this effect being dependent on nanoparticle concentration, sizes and properties of the surfaces. One of possible mechanisms of this effect can be based on blood plasma proteins adsorption on the surfaces of diamond nanoparticles that we have demonstrated in our experiments.