Objectives: Sarcopenia which can lead to decline in physical ability has been known as risk factor on mortality and morbidity. However, little studies have found the effects of muscle mass on mortality of patients with Acute Kidney Injury (AKI) requiring Continuous Renal Replacement Therapy (CRRT).  Methods: We collected 2,003 AKI patients who received CRRT in 8 medical centers between 2006 and 2021. The skeletal muscle areas (SMA) with a threshold of −29 to 150 Hounsfield units from CT images at the level of the 3rd lumbar vertebra was obtained through automated software at Asan medical center. SMA was further categorized in normal attenuation muscle area (NAMA) and low attenuation muscle area (LAMA) to assess the density of muscle. We used Cox proportional hazard model to investigate the association between 30-day mortality and skeletal muscle index (SMA, NAMA, and LAMA), adjusted by medical centers, diagnosis year, sex, age, albumin, hemoglobin, PT INR, c-reactive protein, and history of hypertension.  Results: More than half of the patients (60%) were male and the mean age of patient was 66.12. The 30-day mortality rate was 53% (n=1,066). An IQR increase of SMA (38cm2) was associated with decreased mortality risk (Hazard ratio [HR]: 0.80, 95% confidence interval [CI]: 0.64-0.99), and HRs were lower in body mass index (BMI)-adjusted skeletal muscle index (SMA/BMI). In subgroup analyses on muscle quality, we identified the 24% decreased risk of LAMA on mortality (HR: 0.76, 95% CI 0.63-0.90) while non-significant effects were found in association between NAMA and mortality (HR:1.07, 95% CI: 0.86-1.34). Conclusions: We found the protective effects of muscle mass on mortality of AKI patients requiring CRRT. In addition, even if the density was low, the effect of muscle mass itself on lowering mortality was significant.