In this work, an oxygen gas concentration controller circuit is designed and implemented for use in medical applications such as incubator environments. Since the air of the incubator environment is dried during the control processes, oxygen gas flow is humidified by the designed ultrasonic nebulizer. A polarographic type oxygen gas sensor was used. A signal conditioning circuit was developed to capture the required voltage levels. Control processes have been performed by a high speed PIC microcontroller. Set values can be entered either via an embedded control circuit or via a keyboard. The system is linear, sensitive, and relative error is only about -1 O-2%. All of the parts of the system are renewable. As a result, a low cost and effective microcontroller based circuit, modular in structure, has been provided.