This study is aimed at investigating the accuracy of four existing theories on the cutoff diameter of cyclones, including Lapple (1950), Stairmand (1951), Barth (1956) and Iozia and Leith (1989), and six on the collection efficiency of cyclones, including Lapple (1950), Barth (1956), Leith and Licht (1972), Dietz (1981), Li and Wang (1989), and Iozia and Leith (1990). The predicted collection efficiency curves of the six different theories were compared with the experimental date in the literature. The comparison shows that different sets of experimental data agree better with a particular theory than the other theories and that none of the theories fits all the available experimental data perfectly well. The experimental data for the cutoff aerodynamic, Dpa50, were further plotted in the log-log coordinate of √CDpa50/D, versus Ref and found to form a straight line (C: slip correction factor, D: cylindrical diameter of cyclone, Ref: flow Reynolds number based on the radius of the cyclone minus the radius of the exit tube). In the plot, the experimental data were compared with four different theoretical predictions on the cutoff aerodynamic diameter. It was found that these four theories are more accurate when Ref is between 10000 and 100000 and that none of them is better than the others.