The present paper discusses the problem of examining ability to mentalize in the psychiatric and psychotherapeutic diagnosis process. It is proved by the research results indicating significance of mentalization in the psychosocial functioning of an individual, the relation between the level of this ability and the appearance of the specific mental disorders and problems, and its role in the treatment process. The paper describes and analyses the classic tool for studying mentalization – Reflective Functioning Scale. The present paper points to the advantages of this tool – good reliability, high criterion and theoretical validity, enabling studying complexity of a phenomenon, and introduction of non-specific therapeutic factors into the diagnostic process. The limitations of the tool are also mentioned – high cost borne by a researcher and an examined person, the complexity of the interview collection and analysis procedure and the inability of repeated application of the tool to evaluate treatment effects. The paper also presents an alternative method of studying mentalization that uses questionnaire tools. The text describes the advantages of the questionnaire in assessing a reflective function: first and foremost, its low cost and an easy repeatability of the test. The paper also shows the limitations of this tool in describing the subject phenomenon: inability to describe the complexity of the phenomenon, simplified results and low theoretical validity.