This paper is devoted to construction of a mathematical model for determining the distribution function of blood cells by size. After considering the problem of light scattering on a spheroidizied particle with a multilayer structure and arbitrary oriented in space, the reflection coefficient of a plane wave from the biostructure model with a smoothly irregular structure has been found. This approach made it possible to construct a model of the said structure with several layers. Based upon the reflection of a Gaussian beam from this structure the distribution function of blood cells by size was obtained. Some methods such as the T-matrices apparatus, the Huygens – Fresnel transform, the Tikhonov's regularization were involved. The mathematical model proposed for the first time allowed theoretical calculation of the size distribution functions of spheroidizied particles simulating blood cells for the case in vivo.