This study focused on a group of Chinese adolescents and examined whether the degree of alexithymia would mediate the effect of PTSD from past trauma onto psychiatric co-morbidities and whether gender differences would moderate the mediational effects of alexithymia. Three hundred and twenty-six adolescents were recruited from two schools and completed the Posttraumatic Stress Diagnostic Scale, General Health Questionnaire-28 and Toronto Alexithymia Scale-20. The results showed that 54 % had no trauma in their lives; 10, 21 and 15 % met the criteria for full, partial and no-PTSD respectively. After adjusting the number of traumatic events, difficulty identifying feelings mediated the path between PTSD from past trauma and psychiatric co-morbidity. Gender moderated the mediational effect of difficulty identifying feelings. To conclude, adolescents can develop PTSD symptoms and psychiatric co-morbidity following exposure to a traumatic event. For both males and females, difficulty getting in touch with feelings can influence the impact of PTSD onto psychiatric co-morbidity.