The aim of this study, as an outcome/process research, is to examine the applicability and efficacy of Transdiagnostic Cognitive Behavior Therapy on reducing signs and symptoms of Iranian patients with BPD and co-occurring emotional disorders. Inclusion criteria are: a) age above 18, b) Borderline Personality disorder (BPD), and c) another emotional disorder as a comorbid disorder. Exclusion criteria are: a) presence of diagnostic criteria for psychotic disorder/bipolar disorder/ drug abuse or dependency in the time of research, b) risk of imminently suicidal attempt. By purposeful sampling method, six volunteer patients with Borderline Personality Disorder who met in/exclusion criteria, will participate in a multiple baseline experimental single case study. Participants are recruited from Clinical Psychology and Psychiatry Clinics of Taleghani Hospital and Counselling Center of Tehran University, Tehran, Iran. Each participant will complete the Borderline Personality Inventory (BPI), the Difficulties in Emotion Regulation Scale (DERS), the Positive and Negative Scale (PANAS), the Work and Social Adjustment Scale (WASA), the Affective Lability Scale (ALS), the Toronto Alexithymia Scale (TAS) during 3 baseline, 8 treatment modules and 1 follow up assessments sessions. The unified protocol will be carried out for 16-20 weekly 50 minute sessions. The expected conclusion is deduction of BPD and comorbid disorders symptoms and improvement of emotion regulation skills.