Background: Inadequate supply of protein and energy results in malnutrition. Critically ill patients have high energy expenditure and consequently, require high energy nutrition, also they lose muscle, besides the effects of a procatabolic hormones and cytokines. This study aimed to evaluate the impact of proteins' supplementation during the critical illness of patients in ICU, on muscle thickness (cm) on ultrasound, ventilator need, and two-month mortality. Methods: Our prospective comparative study involved 60 cases who were suffering from acute critical illness and had parenteral nutrition during their ICU stay. The patients were divided into 2 groups; a standard protein group who received a protein concentration of 1 g/kg/day (Group A) and a high protein group who received a protein concentration of 2 g/kg/day (Group B). The nutrition was delivered through a central line and separate bottles technique. Results: The muscle thickness of the forearm and thigh, besides overall muscle thickness, was significantly higher in group B on day 7. The duration of mechanical ventilation was not significantly different between both groups. The protein dose was not significantly associated with overall two-month mortality. Conclusion: The supplementation of 2 g/kg/day of protein parenterally, exhibited significant improvement of muscle thickness, ventilator need, and mortality rates. Studies with larger sample size and longer duration of follow-up are recommended.