A miniature cyclone with four inlets was designed and experimentally calibrated in this study as a size-selective inlet for compact/miniature sensors monitoring ultrafine particles (UFPs). The development of compact/miniature UFP sensors stemmed from the concern for adverse health effects resulting from ultrafine particles, reported by recent epidemiological studies. The performance of these sensors was often interfered by the presence of large particles in sampling stream. Miniature cyclones with multiple inlets can also minimize the potential issue of directional aerosol sampling, encountered in single-inlet cyclones. The pressure drop and particle cut-off performance of this quadru-inlet cyclone were experimentally characterized at various aerosol flowrates. Compared with mini-cyclones developed by Hsiao, Chen, and Son (2009) it is found that the prototype has smaller cutoff particle size and lower pressure drop when operated at the same aerosol flowrate. The linear relationship between the dimensionless cutoff particle size ( i . e . , C 0 . 5 d 50 D C ) and the annular flow Reynolds number (Reann), reported for single-inlet cyclones, was also observed for the studied quadru-inlet cyclone. Semi-empirical models were further proposed to calculate the pressure drop and cutoff particle sizes of mini-cyclones with only one-, two- and four inlets opened. The proposed models may serve as the guideline for the future design of mini- cyclones with multiple inlets.