Abstract: Objective: To evaluate the efficacy and safety in treatment of malignant hydrothorax with Ribonucleic Acid intrapleural injection. Methods: 62 patients of treatment group were divided into two groups. 32 patients of treatment group were treated with Ribonucleic Acid 100mg intrapleural injection. 30 patients of control group were treated with DDP 60~80mg intrapleural injection. After the analysis of date, comparing the efficacy and safety of the two groups. Results: The effective rate of treatment group was 90.60. The control group was 60.00. They had a very significant difference between the two groups (P<0.01), The cases of the clinical benefit response of KPS was 27(81.3%) in the treatment group and 16 (50.0%) in the control group. They also had a very significant difference (P<0.01). The major toxicity included transient fever in the treatment group and bone marrow suppression, Nausea and vomiting in the control group. The difference was very significant too (P<0.01). Conclusions: Ribonucleic Acid has a higher effective rate and a less toxicity in treatment of malignant hydrothorax. The reticular pleural adhesion is uniform and extensive. Ribonucleic Acid can improve appetite and quality of life of patients. Is worth of clinical popularizing.