Electrospun nylon-6 nanofibrous webs with fiber diameters in the range of 30–110 nm were prepared and employed as a membrane material for water filtration due to their excellent chemical and thermal resistance as well as high wettability. They separated all particles with sizes from 10 μm down to 1 μm whereas approximately 90% separation was obtained for 0.5 μm particles. The low content of particles in the filtrate, indicated by the high separation factor, and the low average flux were observed when a thick membrane was employed during the separation process. This can be attributed to partial particle-trapping throughout the membrane thickness, to the longer tortuous flow path and to the larger surface area of the thicker membrane. Furthermore, the smaller the particle size, the lower were the flux and separation factor. As particles were extremely small, they were able to pack closely together on the membrane surface giving a so-called “layer effect” and thus reducing the effective pore-size of the electrospun membrane significantly at the surface. This dense “cake” acted as the separating layer for the electrospun membrane, resulting in unusually high separation factor for high content of ultra-small particles. The fouling mainly occurred within the surface of the electrospun membrane since particles were mostly retained on the surface and only slightly within and at the bottom of the membrane.