Polysulfone (PSU) is one of the most frequently used polymers in membranes technology, and it can be electrospun to form nonwoven fibers. However, its hydrophilicity should be improved for water purification purposes. The fiber diameter in electrospun membranes can affect the physical properties of the product. The electrospinning parameters can largely control the average fiber diameter in electrospun membranes. In this study, PSU/SiO2/TiO2 nanocomposite membranes were fabricated via the electrospinning method, and the effect of the introduced nanoparticles on the properties of the primary solutions and hence the average fiber diameter has been investigated. Silica and Titania nanoparticles (0, 0.5, 1wt%) were added to 20 wt.% PSU solution and were electrospun at a constant voltage and feed rate of 15.5 kV and 2 ml/h, respectively. The conductivity of the prepared solutions and the surface tension was measured. In addition, the rheological behavior of the solutions was evaluated using rheometric measurements. Finally, scanning electron microscopy (SEM) was used to study the morphology of the membranes and estimate the average fiber diameter. The obtained results showed that increasing titania nanoparticles at a constant SiO2 content has decreased the average diameter from 959 to 712 nm, which can be attributed to the higher conductivity and lower surface tension of solutions with a higher TiO2 amount leading to facilitated fiber stretching during electrospinning. On the other hand, introducing silica nanoparticles to a certain extent can decrease the fiber diameter due to the predominant effect of improved conductivity. However, beyond this extent, the significant increase in the solution viscosity at 1wt.% caused the formation of thicker fibers with an average diameter of about 1160 nm. These findings emphasize the importance of solution properties for designing an efficient water membrane by controlling its fiber diameter and specific surface area.