question: Coronavirus Disease (COVID-19) affects the respiratory system in affected patients, so treatment is needed in the form of respiratory assistance, both invasive and non-invasive. The ICU ventilator system is one of the breathing apparatus that works invasively where its use is by inserting a breathing apparatus through the patient's mouth or nose. The fraction of inspired oxygen (FiO2) is one of the variables that must be regulated in an ICU ventilator system because it affects the level of oxygen that enters the human body. Mixing of air and oxygen gas is done automatically using PID control which is applied to the proportional flow control valve of air and oxygen. This paper discusses the control of PID-based Fraction of inspired oxygen implanted in the ICU Ventilator. Adjusting the ratio of airflow and oxygen mixed in the mixing tank will produce airflow with the desired fraction of inspired oxygen. Teensy 4.0 Microcontroller and the C programming language are used to execute the PID control algorithm. In this study, the best Kp, Ki, and Kd values were determined using the PID Tuner, for the air proportional flow setpoint and oxygen proportional flow setpoints of 30, 60, and 100 LPM, respectively. These values are able to produce precise and accurate control of the FiO2 according to the desired setpoint on the ICU Ventilator. The results of the FiO2 test obtained an average value for rise time of 13478 ms, overshoot of 0.11%, settling time of 19803 ms, and steady-state error of 1.15%. Meanwhile, the results of the inspiratory flow test obtained an average value for a rise time of 964 ms, an overshoot of 5.43%, a settling time of 4697 ms, and a steady-state error of 0.98%. © 2021 IEEE.