Although it has been reported that 25% to 50% of patients with eating disorders engage in self-harming behaviors (SHBs), including nonsuicidal self-injury (NSSI) and suicidal behavior (SB), no study has investigated the psychological mechanisms underlying these SHBs or any differences that may exist between NSSI and SB. This study involved 76 female patients with eating disorders who were treated at the Kyoto University Hospital between July and August, 2010, who answered questionnaires about SHBs, eating attitudes, tendency to dissociate, and attachment style. Some of the participants (22.4%) had other psychiatric disorders in addition to eating disorders, including borderline personality disorder, dissociative disorder, and posttraumatic stress disorder. Of the participants without comorbidity, 23.7% had engaged in SHBs in the past 3 months. Participants with comorbidity tended to dissociate significantly more than participants without comorbidity. Logistic regression indicated that, in all participants, NSSI in the past 3 months was related to the tendency to dissociate and having a higher body mass index, whereas preoccupied attachment style was potentially related to recent SB. Specifically, among the participants without comorbidity, a sense of ineffectiveness and poor interoceptive awareness were related to recent NSSI, and severity of binge-eating was related to SB. In contrast, a history of NSSI in the patients with eating disorders without comorbidity was related to a sense of ineffectiveness. The results of our study suggest that it may be important to help patients with eating disorders recover their own sense of effectiveness as a possible way to reduce SHBs.