The instability of a composite material reinforced with a periodic row of parallel short fibers is studied considering the interaction of neighboring fibers. Emphasis is on the mutual influence of short fibers in the matrix during loss of stability, depending on the distance between them. A piecewise-homogeneous medium model and the three-dimensional linearized theory of stability of deformable bodies are used Keywords: composite materials, nanocomposites, short fibers, nanotubes, three-dimensional linearized theory of stability, inhomogeneous stress-strain state, finite-difference method, interaction of fibers, critical strain, instability modeIntroduction. As seen during recent development of the composite technology, composite materials can exhibit very interesting combinations of major properties such as high strength, high-temperature range, oxidation resistance, fatigue strength, etc. Fibrous materials such as carbon fibers, oxide fibers and whiskers, and carbides used as reinforcements reduce the weight of composites, preserving their strength. This is why the greatest success in the use of composites was achieved in aerospace technology, manufacture of gas-turbine engines, high-speed cars, extreme yachts and raceboats, etc. The major factors that limit the use of most composites are the high cost of reinforcement fibers and technology-related difficulties that do not allow gaining the greatest benefit from the strength of reinforcing fibers in structural elements made of composites.Composites owe their strength to the combination of two materials with different mechanical properties. For maximum strength, the harder component should play the role of reinforcement. Reinforcing elements should be long enough to provide strong bonding with the matrix. Therefore, the most favorable form of reinforcement is a thin fiber because, as is well known, the thinner the fiber, the stronger it is. However, continuous, very long fibers cannot always be used for technological reasons since the geometry of too many products is such that they cannot be made of continuous fibers. Moreover, very long fibers cannot be made of all available materials and fibers should be laid up so as not to reduce their strength. For example, Fig. 1 [2] shows an end of a TaC-(Co+Ni-Cr) unidirectional eutectic alloy pickled after crystallization on which monocrystal whiskers can be seen.The recent years saw the advent of nanocomposites, a new class of composites. Their reinforcing elements measure several nanometers. A nanocomposite is a material consisting of a matrix and nanoparticles (nanotubes) as a reinforcement (Fig. 2 [13]). This type of structure is in full compliance with the terminology of the micromechanics of composites with polymeric or metal matrix.Consider a simplified nanotube model. A perfect nanotube (CNT) is a seamless cylinder formed by rolling a flat hexagonal graphite sheet. Such a tube does not form seams upon rolling and has hemispherical caps at the ends, which include six regular pentagons, along with regular hexagons...