The urban environment contains several sources of particulate matters (PM). The street dust (SD) summarizes particles from all PM sources in urban area. Our present research is focused on the street dust collected form Piata Garii in Cluj Napoca in autumn. The particle distribution analysis shows that 60.84 % of the collected SD is gross (over 200 μm diameter), only a small fraction of 0.83 % is situated below 64 μm diameter. Even a small rate of this could be harmful considering the street dust amount reported for all square surfaces. The mineral components were identified by X-ray diffraction: quartz, muscovite, calcite, kaolinite and lepidocrocite. Finest particles were trapped on the solid substrate (e.g. glass) by adsorption from a SD dispersion in deionized water. The obtained PM film was investigated both by AFM microscopy and X-ray diffraction. Our findings suggest that a thin layer of nano – scaled PM containing finest quartz particles and finest clay fragments is adsorbed on the glass. The average diameter of nano – scaled PM is situated at 95 nm for quartz and around 45 nm for clay particles. As a final remark all these ultra fine PM are sensitive to humidity proving the cohesive tendency to form bigger particles. This could be a way to reduce their propagation in air.