The diagnostic phase is the essential part of the consultancy cycle, providing the client with interpretable data and systematic approaches to improve the consultation effectiveness, however, there are still areas for improvement in diagnostic frameworks, data analysis, and client-consultant communication. This paper will demonstrate that although the diagnosis is an ongoing activity and a crucial node linking the whole consultancy cycle, it still requires using a mix of context-specific empirical tools and fostering a sustainable client-consultant relationship, dedicated to exploring the root causes of the organizational issue. In order to explain this, the paper will firstly focus on the importance of the diagnosis. Secondly, it will evaluate the diagnostic methods and data collection ways. Thirdly, it will discuss how to provide effective feedback to clients and build strong client-consultant relationship in the diagnostic phase. Finally, the paper will summarize the critical study of the diagnosis.