Aerosol nanoparticle deposition onto a surface under a temperature gradient is commonly applied in the chemical and medical industries. In this study, a numerical investigation with a two-phase model is used to investigate the deposition characteristics of nanosized particles in a 90° square bend. The effects of variations in the gas phase physical parameters, such as density, viscosity, and thermal diffusivity with changing temperatures are studied. The main forces acting on the particles are the drag forces, Brownian forces, and thermophoretic forces. A discrete phase model (DPM) based on the FLUENT software is used to investigate particle transfer. The results show that in a temperature gradient flow, particles move towards the colder wall, and some of them strike and deposit onto its surface. The particle deposition efficiency increases with the temperature gradient rising. The Brownian force plays a more important role in particle deposition when smaller particles are used. Because of inertia and gravity, particle deposition on the four surfaces of a 90° square bend tube is inhomogeneous. The deposition efficiency on the floor surface increases with increasing particle diameter. On the contrary, larger particles decrease the deposition efficiency on the ceiling surface.