Objective: The aim of this study was to investigate the course of diaphragm changes in mechanically ventilated patients in ICU after four days of mechanical ventilation. In addition, to evaluate correlation of various demographic variables to baseline diaphragm muscle thickness and their effects on the course of diaphragm muscle thickness changes. Methods: This study was conducted from December 2018 to February 2019 at Cipto Mangunkusumo National Hospital. Thirty critically ill patients using mechanical ventilation in the ICU were included. Baseline demographic data were collected. Baseline end expiratory diaphragm thickness was measured within one hour after starting mechanical ventilation and repeated every 24 hours for four days. Mortality and duration of mechanical ventilation were recorded. Results: There were no differences in baseline diaphragm thickness according to gender, body mass index and modified Rankin Score. The subjects were divided into two categories: duration of mechanical ventilation less than seven days and duration of ventilation of seven days or more. The mean baseline diaphragm thickness was 2.11 + 0.15 mm. A rapid decrease of diaphragm muscle thickness was found within 24 hours. Diaphragm muscle thickness changed over subsequent measurements during observation. It was found that rapid diaphragm muscle thinning corresponded with longer duration of mechanical ventilation and worse outcomes. Conclusion: Diaphragm muscle thinning was seen early in the course of mechanical ventilation. Diaphragm muscle thickness changed over time, and there was no correlation with other measured variables. © 2021 Pakistan Medical Association. All rights reserved.