Increase in the number of juvenile offender has alerted the society, mainly adult that there are some possible risk factors that may lead to this emerging problem. Numerous studies had been conducted to explore the risk factors and mostly suggested mental health problems as the biggest part in problems among juvenile. One of the most common mental health problems found among juvenile was personality disorders. Personality disorders were strongly suggested as a precursor to unstable and aggressive behaviors among juvenile offender. For methodology, a cross-sectional study was conducted aims to explore two types of personality disorder; antisocial personality disorders (APD) and borderline personality disorders (BPD) among juvenile, and examine the effects of personality disorder on aggressiveness. Participants (male and female juveniles) came from a detention school in Peninsular Malaysia. The participation was based on convenience sampling. The findings showed that APD was high among both groups, whereas BPD was high among girls only. Independent t-test demonstrated higher mean for APD among boys, whereas girls had higher mean for BPD. Only BPD was found significantly different across the gender (p < .05). Simple regression analysis showed that among boys, APD was significantly related to physical (PA) and verbal aggression (VA), whereas BPD was significantly related to PA, VA, and hostility. Among girls, APD was found significantly related to PA and anger, whereas BPD had no significant relationship to any scales of aggression. In conclusion, there is prevalence of APD and BPD among juvenile, and both significantly affect juvenile’s aggressive behavior.