Mentalization is the capability of individuals to understand the mental states of themselves and others. Patients with Borderline Personality Disorder (BPD) are characterized by deficient mentalization abilities, which result in impaired social functioning and distorted self-identities. With the acknowledgement of the importance of mentalization for BPD patients, this review aims to summarize the possible causes of mentalization deficiency among BPD patients. A systematic review is conducted using EBSCO Discovery Service (EDS), EBSCOhost APA PsycARTICLES, and Google Scholar for the period between 2010 and 2020. The descriptors used are 'borderline personality disorder' and 'mentalization'. From the neurological aspects, when performing social cognition tasks, BPD patients display the usage of a different brain pathway characterized by the hyperactivation in amygdala and hypoactivation in brain areas related with Theory of Mind. From the genetic aspects, certain gene alleles are possible determinants for the impaired mentalization abilities. From the psychological aspects, the mentalization deficiency is affected by early childhood experiences, especially abuses. These result in the later diagnosed borderline personality disorder and damage-related psychological functions. It is concluded that the mentalization deficit among BPD patients can be traced back to genetics, neurological causes, early childhood experience, and psychological causes. And these findings may contribute to the diagnosis, treatment, and early prevention of Borderline Personality Disorder.