Muscle volume measurement is important for ICUAW evaluation, which has been done using ultrasound and manual muscle testing. We compared the reliability of CT and other methods for muscle volume measurement. This observational study assessed 7 patients 40 years old or older who had been admitted to our ICU. Based on a prespecified CT protocol, muscle volume was evaluated on admission day and at 10-14 days after admission. Results show that the femoral muscle volume decreased significantly by up to 20%. The ICC between two raters was 0.97. Psoas muscle volume decline did not correlate with the femoral muscle decline. Femoral muscle volume evaluation was reliable and objective. This method is useful to quantify the ICUAW severity. Our pilot study based on CT femoral muscle volume evaluation will facilitate further studies to prevent ICUAW. Results show that CT using femoral muscle volume measurement is a reliable means of ICUAW evaluation.