An more reliable human upper respiratory tract model that consisted of an oropharynx and four genera- tions of asymmetric tracheo-bronchial (TB) airways has been constructed to investigate the micro-particle deposition pattern and mass distribution in flve lobes under steady inspiratory condition in former work by Huang and Zhang (2011). In the present work, transient airflow patterns and particle deposition during both inspiratory and expiratory processes were numerically simulated in the realistic human upper respi- ratory tract model with 14 cartilaginous rings (CRs) in the tracheal tube. The present model was validated under steady inspiratory flow rates by comparing current results with the theoretical models and pub- lished experimental data. The transient deposition fraction was found to strongly depend on breathing flow rate and particle diameter but slightly on turbulence intensity. Particles were mainly distributed in the high axial speed zones and traveled basically following the secondary flow. “Hot spots” of deposition were found in the lower portion of mouth cavity and posterior wall of pharynx/larynx during inspira- tion, but transferred to upper portion of mouth and interior wall of pharynx/larynx during expiration. The deposition fraction in the trachea during expiration was found to be much higher than that during inspiration because of the stronger secondary flow. An more reliable human upper respiratory tract model that consisted of oropharynx and four genera tions of asymmetric tracheo-bronchial (TB) airways has been constructed to investigate the micro-particle deposition pattern and mass distribution in flve lobes under steady inspiratory condition in former work by Huang and Zhang (2011). In the present work, transient airflow patterns and particle deposition during both inspiratory and expiratory processes were numerically simulated in the realistic human upper respi- ratory tract model with 14 cartilaginous rings (CRs) in the tracheal tube The present model was validated under steady current respiratory rate rates and particle diameter but slightly on turbulence intensity. Particles were mainly distributed in the high axial speed zones and traveled basically followin g the secondary flow. “Hot spots ” of deposition were found in the lower portion of mouth cavity and posterior wall of pharynx / larynx during inspira tion, but transferred to upper portion of mouth and interior wall of pharynx / larynx during expiration The deposition fraction in the trachea during expiration was found to be much higher than that during inspiration because of the stronger secondary flow.