This study examined whether training can increase the reflective function (RF) of novice therapists about patients with Borderline Personality Disorder (BPD). A total of 48 students in clinical psychology were randomly assigned to mentalization training or didactic training. Their RF regarding patients was assessed with the Therapist Mental Activity Scale (TMAS: Normandin, Ensink, & Maheux, 2012). The RF of trainees assigned to the mentalization training improved significantly, while participants who received traditional didactic training actually became significantly less reflective. These findings show that brief mentalization training can help beginner therapists develop their mentalization capacities with challenging patients.