Nosocomial infection represent a significant cause of patient morbidity and mortality, especially in the ICU and nosocomial pneumonia that cause high mortality may arise from contaminated respiratory equipments. This study was done to investigate the relationship of micro-organisms in upper respiratory tract and ventilatory equipment, especially humidifier. The total number of cases in this study was 20 and the specimen for bacteriologic culture were taken from the pharynx, the distilled water and the inner wall of humidifier and toilet of trachea at the 0, 4, 8, 12, 24, 48 hours after initiation of mechanical ventilation. The results obtained from the bacteriologic culture were as follows. 1) The most frequently found organism at the pharynx was Pseudomonas aeruginosa and next was Coagulase Negative Staphylococcus, followed by Streptococcus viridans, Klebsiella pneumoniae, Staphylococcus aureus, Streptococcus group D, Enterobacter agglomerans, Acinetobacter spp. 2) The most frequently found organism at the trachea was Pseudomonas aeruginosa and next was Klebsiella pneumoniae followed by Coagulase Negative Staphylococcus, Streptocoecus viridans, Pseudomonas picketti, Staphylococcus aureus and no growth in 5 cases. 3) The most frequently found organism in the fluid of humidifier was Pseudomonas aeruginosa and next was Pseudomonas picketti, Klebsiella pneumoniae, Streptococcus group D and no growth in 2 cases. 4) The most frequently found organism at the pharynx was Pseudomonas aeruginosa and next was Pseudomonas picketti, Klebsiella pneumoniae, Streptococcus group D, Serratia liquefacines, Acinetobacter spp and no growth in 3 cases. 5) The number of isolated organisms in the fluid of humidifier was 3 at 0 hour and 18 after 48 hours and at the wall 2 at 0 hour and 15 after 48 hours. 6) The number of same organism that was isolated between the pharynx and trachea was 13 among 15 cases and between the wall and fluid of humidifier was 15 among 16 cases and between trachea and fluid of humidifier was 10 among 14 cases.