We usually use spirometry for the medical follow-up of respiratory mechanics after lung transplantation. However, especially in the first few post-operative weeks, it is easily affected by postoperative pain and the patient's co-operation during forced breathing effort. To avoid missing out on assessing pulmonary function, we perform non-invasive forced oscillation techniques on the patients who cannot perform forced breathing maneuvers. In this paper, we discuss the application of forced oscillation techniques on a patient with suspicion of acute lung rejection, whose spirometry could not be correctly performed and seemed to be unreliable. The respiratory impedance measurements had good correlation with the patient's clinical symptoms before and after steroid therapy. Thus, postoperative pulmonary function follow-up using forced oscillation technique was useful in assessing peripheral airway condition in critically ill patients, and may be able to detect acute rejection.