The tracheobronchial histopathologic findings in 7 healthy cats used with high frequency oscillation ventilation (HFOV) were compared with those in 6 cats used with conventional mechanical ventilation (CMV). 4-point, 9-variable scoring system was used to evaluate the injury in the trachea, right & left main bronchi and parenchyma. The following results were obtained; 1) The tracheobronchial tree received HFOV had no significant damage compared with CMV (P>0.05). 2) Intraepithelial mucus loss and emphysema were slightly more prominent in CMV groups. As above results; the tracheobronchial histopathologic difference was not prominent between CMV and HFOV groups received with relatively short period, however, the cellular of function and barotrauma may be more prominent in CMV groups. From now on, as causes of tracheobronchial injury in HFV, interaction between humidification and mechanical trauma considers further study.