The deposition of polydisperse particles under the influence of gravity is examined using computer simulation. A parameter, σ, that represents the standard deviation of particle size is used for studying the effect of the variation in polydispersity on the resulting microstructures. Structural correlations are examined through contact networks, radial and angular distribution functions, and diffraction patterns. The results show that the onset of ordering appears near σ=0.05 as σ is decreased. The long-range ordering of the structures is not influenced by the introduction of a small amount of polydispersity, which may increase the uniformity of local density distribution in the angular direction. Polydisperse systems with small deviations in size display stronger positional order in some directions and this in turn contributes to the uniformity of overall packing structures.