In about 25-40% of pleural effusion cases not reached to diagnosis, so taking a pleural biopsy may decrease the number of cases with difficulty in diagnosis of pleural effusion. Using pleuroscopy has high accuracy in the diagnosis of these cases. It is safe with complications. In this study, 60 patients who underwent a pleural biopsy by pleuroscopy were studied with unknown causes of pleural effusion. The mean age was (51-/+13.2) and male to female ratio was 4 to 1. There were 36.6% of cases diagnosed with tuberculosis, 23.3% of cases diagnosed with adenocarcinoma. Pleuroscopy failed to reach a diagnosis in 16.6% only. So, the pleuroscopy yields high accuracy in the diagnosis of pleural effusion with unknown causes.