Abstract: For pharmaceutical industries, clinical data is one of the most valuable deliverables. It is also the basis of analysis, submission, approval, labeling and marketing of a drug product. To ensure the integrity and reliability of clinical data, a scientific standardized quality control (QC) has to be established at each step of a clinical trial. Data validation is conducted to ensure the reasonability and compliance of clinical data by checking data quality before the data is statistically analyzed. This paper focuses on purpose of data validation, creation of data validation plan, rationale of data validation, types of data validation and performance of user acceptance testing on clinical database.