Fungal spores can be detached from the ventilation duct and then transmitted indoors. As airflow sweeps the ventilation duct, it may decelerate in the boundary layer, and the resulting spore detachment may vary according to the position of the fungal colony. This study investigated the aerosolization potential of Aspergillus niger spores from the inner surface of four typical ventilation ducts. The airflow near the duct surface was numerically solved to determine the local air speed distribution at the height of the growing fungi. The results revealed that the spores growing at the center of the rectangular duct surface were more easily aerosolized than that growing at the corner. When the inlet air speed was 5.00 m/s, the spores of the 4- and 10-day-old colonies could not be detached when the dimensionless displacements of the circular tube were larger than 0.27 and 2.43, respectively. The aerosolization potential of spores increased with the β value and the constriction angle of the reducer. The maximum value of the local dimensionless air speed for the contraction angle of 50o (4.5) was greater than that for the contraction angle of 20o (4.0). Such information is useful for indoor mold contamination control.