Learning Objective #1: To position the critical ill patient by adequate posture will promote the oxyganation in playing a good nurses' role.

Learning Objective #2: Evidence-base research is important to apply nursing clinical pratice.

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The purpose of this study was to assess the effect of different positions on oxygenation in critically ill patients. Research design was prospective and quasi-experimental, intervened by different positions compared with the lying. 20 participants were excluded when the critical situation was not saturated with one of criteria (APACHII』15, X-ray showed lateral or bilateral pulmonary effusion, PaO2 /FiO2 ratio from 100 to 300, Endo-trachea catheter indwelling or tracheotomy with positive pressure mode ventilator, artery catheter installation, non-limitation in change position and permission by participant). Each patient experienced semi-prone and semi-Folwer・s position, time extended (per 30mins untile 120mins) and physical indicators measurement (PaO2 /FiO2 ratio, SaO2, MBP and PR). Demography of the sample, including the mean of age was 75.2, four times of male to female ratio, the X-ray result showed pulmonary effusion bilateral 18 (90%) and single right side only 2(10%) during eight months data collection. Analysis of variance for repeated measures was used by GEE model for control the individual difference. Outcome indicators approved PaO2 /FiO2 ratio(M=1.8, SD=.55, Coef.=0.223, p<0.01) difference increased significantly by time extended to 120 minutes on Semi-Fowler・s position, MBP (M=81.5, SD=10.37, Coef.=-5.60, p<0.05) showed the significantly negative direction for stabilization on the Semi-Fowler・s when other position with time extended to 120mins increased. SaO2, HR and RR were no significant difference within total process. These findings suggest that Semi-Fowler・s positioning of critically ill patients who had pulmonary effusion have higher PaO2 /FiO2 ratio and lower mean blood pressure when time extended to 120mins does not further endanger other vital signs. Evaluation of individual patient responses to position changes in the clinical setting is encouraged until further studies using more heterogeneous.