The purpose of this study was to identify the mannerism phenomenon, which psychiatric nurses experience in hospitals. Methods: Colaizzi`s phenomenological research methodology was used to illuminate the meaning and nature of the experience inherent for nurses when they provide nursing service. Results: Analysis of the data collected through interaction with the participants showed five theme clusters, as follows: `Losing passion`, `Becoming impassive mien`, `Watching and folding up one`s mind`, `Assimilating into the situation` and `Being aware of change to one`s existing style.` Conclusion: The results of this study provide a description of the phenomenon of mannerism as described by the participants, and allow for identification of the nature of the meaning of the mannerism experience. Accordingly, the study findings provide psychiatric nurses an opportunity to meditate on the nature of nursing in high-quality nursing service for patients with mental illnesses. Further, medical authorities need to provide the opportunity for nurses to apply nursing intervention therapy as part of nursing service and establish an institutional, administrative system to make it possible for nurses to provide service.