Purpose: The purpose of this study was to investigate the experience of horizontal violence in intensive care unit (ICU) nurses. Methods: This is a methodological triangulation study consists of quantitative and qualitative research methods. A total of 134 ICU nurses from 5 hospitals participated in a survey on nurse to nurse horizontal violence, and the qualitative data were collected through 3 focus group interviews. Results: 94.0% of the participants have experienced horizontal violence within the past 6 months. The question with the highest mean score was "dominating senior nurses mood", and the question with the lowest mean score was "physical assaults". The qualitative data analysis revealed 17 themes on the backgrounds, types, and influence of horizontal violence. Conclusion: The survey result shows that the problem of horizontal violence in ICU nurses is serious, and the focus group interview extracts additional types of horizontal violence such as "scolding in a wrong way", "pressing to resign", "leaving out in the cold", and "burning". Further research needs to be done to build a multidimensional model of horizontal violence in the nursing workplace.