Mentalization is the capacity to perceive and interpret behavior in terms of mental states of self and other. The purpose of this study was to develop the questionnaire about mentalization (Mentalization Questionnaire; MQ), and to examine the relationship between mentalization and tendency of Borderline Personality Disorder (BPD). The university students (n=181) completed the MQ and the BPD scale. Factor analysis of the MQ revealed two factors. Internal consistency and test-retest reliability for the MQ showed sufficient reliability. The results showed that both Mentalization of self and Mentalization of other in the MQ were negatively correlated with items about instability of self-image, emotion regulation, interpersonal relationship, and, tendency of self-harming in the BPD scale. Participants with high BPD tendency was found to show significantly lower score of MQ than participants with low BPD tendency. These results indicated that the MQ had sufficient reliability and validity and the MQ is useful for measurement of individual differences of mentalization.