OBJECTIVE: The distinguishing features of Bipolar I Disorder (BD I) from Bipolar II Disorder (BD II) may reflect a separation in enduring trait dimension between the two subtypes. We therefore assessed the similarities and differences in personality traits in patients with BD I and BD II from the perspective of the Five-Factor Model (FFM). METHODS: The revised NEO Personality Inventory (NEO-PI-R) was administered to 85 BD I (47 females, 38 males) and 43 BD II (23 females, 20 males) patients. All included patients were in remission from their most recent episode and in a euthymic state for at least 8 weeks prior to study entry. RESULTS: BDII patients scored higher than BD I patients on the Neuroticism dimension and its four corresponding facets (Anxiety, Depression, Self-consciousness, and Vulnerability). In contrast, BD II patients scored lower than BD I patients on the Extraversion dimension and its facet, Positive emotion. Competence and Achievement-striving facets within the Conscientiousness dimension were significantly lower for BD II than for BD I patients. There were no significant between-group differences in the Openness and Agreeableness dimensions. CONCLUSION: Disparities in personality traits were observed between BD I and BD II patients from the FFM perspective. BD II patients had higher Neuroticism and lower Extraversion than BD I patients, which are differentiating natures between the two subtypes based on the FFM.