Abstract: Objective To study the value and evaluate the safety of medical thoracoscopy in diagnosing pleural effusion of unknown aetioloty and identify the disease spectrum. Methods We performed medical thoracoscopy on these patients who were considered diagnosed as pleural effusion of unknown aetioloty which was defined by hydrothorax and other clinical examinations. Results 348 patients were involved. Diagnosis sensitivity of medical thoracoscope was 99.2%, and specificity was 95.3%. Diagnosis were significant different among different age groups. Complications of the patients underwent medical thoracosopy were fever (n = 52, 15.0 %), subcutaneous emphysema (n = 49, 14.1 %), severe complications (n = 3, 0.9 %) were pleural reaction, heart arrest and myocardial infarction. Conclusion Medical thoracoscopy is an standard option for diagnosing pleural effusion of unknown aetiology for its high sensitivity and specificity, but low incidence of severe complications.