Hydrometer tests (ASTM D422-63(2007), Standard Test Method for Particle-Size Analysis of Soils) were conducted to determine particle size distribution (PSD) of soil particles smaller than 0.075 mm (fines). Test standards permit soils with particles larger than 0.075 mm to be tested. We investigated the effect of maximum particle size, Dmax, on the PSD of fines. Two possible effects of Dmax were hypothesized: one is that an increase in Dmax makes fines settle faster, as larger particles could collide and accelerate fines downward; the other is that an increase in Dmax makes fines settle slower, as larger particles may create upward flow relative to the downward movement of smaller particles. The goal of this research is to provide experimental evidence and understanding of the effect Dmax has on the PSD of fines, as such an effect is not currently considered in ASTM D422-63(2007). First, we tested eight samples of a natural soil (NS), each with a different Dmax: 0.075, 0.15, 0.3, 0.425, 0.6, 1.18, 2.36, and 4.75 mm. These tests showed that an increase in Dmax resulted in an increase of percent finer values for particle sizes smaller than 0.075 mm, even though the PSDs of fines of all samples were the same. To verify this observation, we conducted tests on a ground silica (GS) that was much finer than the NS, following the same methodology. Tests on GS resulted in similar PSD curves, suggesting that different Dmax values made no significant difference on the settling of GS fines. A computational fluid dynamics model was developed to simulate sedimentation. It revealed that localized upward movement (upwash) by larger particles caused smaller particles to settle even more slowly. The research showed that particles larger than 75 μm could alter the PSD obtained from hydrometer testing. Such an effect depends on the PSD of fines, which is more obvious in soils with relatively larger particle sizes (such as NS) than those with relatively smaller particles (such as GS).