Abstract <p>One of the current research areas is the development of optical frequency summation and doubling systems in which nonlinear crystals serve as the active element. Gallium phosphide (GaP) in the form of filamentous nanocrystals, which has a high quadratic dielectric constant and is transparent in the visible and infrared ranges, can be regarded as a promising material for such systems. Owing to their shape, these crystals can be effectively integrated into modern integrated photonic systems. A numerical study was carried out on the process of second harmonic generation in GaP nanowires as a function of their geometric parameters and the orientation of the incident radiation. The conditions that provide the highest generation efficiency along the nanocrystal axis were determined. The possibility of second harmonic propagation along the axis of filamentous nanocrystals in air was demonstrated for specific system parameters—namely, the transverse size of the nanocrystal and the angle of incidence of the radiation. An increase in the transverse size of the structures leads to a reduction in the difference between the actual and predicted propagation directions, since the size-related characteristics of the filamentous nanocrystal tend to become more volumetric with increasing diameter. The results can be used in the development of various nanophotonic devices.</p>