Number size distributions, total number concentrations and mean particle sizes have been measured for 70 days at an urban background site in Mülheim-Styrum, Germany with a handheld Partector Pro and a TSI mobility particle size spectrometer (MPSS). The aim of the study was to evaluate the performance of the Partecor Pro against the MPSS. The results show that the size distributions, measured with the Partector Pro agree with the MPSS mostly within ±25% for particle sizes below 113.5 nm, whereas higher, systematic differences were observed for larger particles. The measurement accuracy was shown to be dependent on the geometric mean diameter and the geometric standard deviation of the aerosol. Best results were found for the most abundant size distributions with geometric mean particle diameters ≥30 nm and geometric standard deviations larger than 1.8. The total number concentration, measured by the Partector Pro was found to be in excellent agreement with the MPSS with a slope of the linear fit of 0.9977 and a regression coefficient of R² = 0.9956. The agreement of the geometric mean particle diameter, determined with the Partector Pro and the MPSS was good, but moderately dependent on the particle size distribution. For mean particle sizes between 20 nm and 50 nm, the bias was within ±15%. Higher deviations of up to 30% were observed when the geometric mean particle sizes exceeded 70 nm and when the geometric standard deviations exceeded approximately 2.7.