A numerical method was developed to simulate the compressible flow field and particle trajectory in an API Aerosizer(TM). Experimental particle time-of-flights (tau(TOF)) data using mono-disperse solid and liquid particles were also obtained. Theoretical particle tau(TOF) results were compared with the present experimental data and those of Cheng er al. (1993, J. Aerosol Sci. 26, 501-514). Good agreement was found for solid particles. The original tau(TOF) calibration for the API Aerosizer(TM) was found to underestimate measured tau(TOF) mainly because of the assumption of one-dimensional flow in the nozzle, and incorrect use of the particle drag coefficient in the supersonic flow field. Effects of decreasing ambient pressure and increasing particle density on increasing tau(TOF) were found to be important.