Position changing with blood distribution in different portions of lung can produce V/Q mismatch, and finally extend to oxygenation disruption. Thus, if position changing has done versus of patients pulmonary problem types, not only does not cause oxygenation improvement, but also induce gas exchange impairment, and with provocation of hypoxemia and respiratory acidosis, results complication serious.Goals of this research were determination of semi-fowlers position and its duration effect on the arterial blood gases (ABGs) in patient under mechanical ventilation hospitalized in general ICU.For study goals attainment, 30 ICU patient (22 men and 8 women) under mechanical ventilation was choosen. After recording patients demographic data on questioner, at first an arterial blood sample in supine position, obtained. Then patients immediately had been placed in semi-fowlers position and after 15, 30, 45, and 60minute lying in this position, samples of arterial blood obtained and sent to laboratory Findings showed that semi-fowlers position had not positive effect on oxygenation and gas exchanges. Also, findings certificated that the effect of lying duration (15, 30, 45, 60 minute) in semi-fowlers position on oxygenation and gas exchange wasn't significant.Finally conclusion is that without consideration of lung pathology, semi- fowler's position should not be used because this position not only does not induce oxygen- improvement, but also causes gas exchange impairment, and also create pressure ulcer in susceptible body portion such as sacral and coccygeal.