It is well accepted that airborne particles can induce adverse health effects dependent on the source, composition, morphology and size. Studies indicate that ultrafine particles (diameter < 100 nm) are of specific importance. Therefore, upwind and downwind field measurements of particle number size distributions (14–750 nm), nitrogen oxides, PM10 and PM1 mass concentrations were performed to derive information on sources of those types of particles from motorways. The measurement stations were located at a motorway in a rural area with flat terrain and unhindered air flow situation. The mean particle number concentration was 20,900 #/cm3 downwind and 3,400 #/cm3 upwind of the motorway. The highest total particle number concentration at the downwind station was 141,000 #/cm3. About 90% of these particles were < 100 nm. The measured data were used to derive size-dependent emission factors (EF) using the NOx tracer method. This method is based on listed NOx EF (HBEFA, 2010). The average total particle number EF per vehicle was determined to be 3.5 × 1014 particles/km. The average particle EF was 2.1 × 1014 particles/km and 11.8 × 1014 particles/km for light duty vehicles (LDV) and heavy duty vehicles (HDV). The higher EF for HDV is mainly caused by particles with diameters below 50 nm. The comparison of EF from the literature show the importance of the particle size range investigated. Especially particles at the lower size detection limit contribute to total particle number concentrations and hence determine the EF significantly. In the EURO V directive, particle number emission limits of 6 × 1011 particles/km were set for diesel passenger cars. This value is defined for non-volatile particles > 23 nm. The EF for the given size range (> 23 nm) determined in this study were significantly higher with 1.0 × 1014 for LDV.