The aim of this study was to compare resting energy expenditure (REE) obtained by indirect calorimetry (IC) and Harris-Benedict (H-B) equations, and to examine whether hypocaloric nutrition support could improve protein nutritional status in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD). Thirtythree COPD patients (20 males, 13 females) were recruited and REE was measured by IC. Measured REE (REEm) was compared to predictive REE by H-B equations (REEH-B) and its corrected values. Correlation between REEm and APACHE II score was also analyzed. Patients were randomly divided into hypocaloric energy group (50%-90% of REEm, En-low) and general energy group (90%-130% of REEm, En-gen) for nutrition support. The differences of albumin, prealbumin, transferrin, hemoglobin, and lymphocyte count before and after 7 days nutrition support were observed. Results show that REEH-B and REEH-B×1.2 were significantly lower than REEm (p<0.01). REEm positively correlated with APACHE II score (p<0.05 or p<0.01). After nutrition support, hemoglobin decreased significantly in En-gen group (p<0.05); lymphocyte count in both groups, and transferrin and prealbumin in the En-low group increased significantly (p<0.05 or p<0.01). Our data suggest that 1) these patients' REE were increased; 2) since IC is the best method to determine REE, in the absence of IC, H-B equations (with standard body weight) can be used to calculate REE, but the value should be adjusted by correction coefficients derived from APACHE II; 3) low energy nutrition support during mechanical ventilation in COPD patients might have better effects on improving protein nutritional status than high energy support.