A cost-effective, miniature electrical ultrafine particle sizer (mini-eUPS) has been developed for the future UFP (ultrafine particle) monitoring network in cities with high traffic density and in communities close to either freeways, airports or stationary combustion sources. The mini-eUPS primarily consists of a mini-plate unipolar particle charger, mini-plate differential mobility classifier and a mini- particle Faraday cage with a sensitive electrometer. A custom-made circuit board was designed for the mini-eUPS operation. Additional to the primary function of measuring the UFP size distribution, the mini-eUPS records the temperature, relative humidity, pressure, altitude, timing and location for each measured data set. The operational status of mini-eUPS was also registered for the data quality assurance. The built-in features of self-diagnosis, malfunction reporting and wireless networking make mini-eUPS suitable for the future UFP monitoring network. The prototype mini-eUPS, having 6.5" (L) × 5.0" (W) × 4" (H) in the overall package ~1.0 kg in the total weight, measures the particles in sizes ranging from 5 to 200 nm. The performance of mini-eUPS was evaluated by comparing its measured data with those measured by a scanning mobility particle sizer (SMPS) for lab-generated particles in both unimodal and bimodal distributions, and UFPs emitted from a low-cost 3D printer and diesel engine. Reasonable agreement between mini-eUPS and SMPS data was obtained in the comparison.