Teacher stress has been a major concern among researchers as it has negative impact on teaching profesion. This study aimed to test the mediating effect of irrational beliefs on the relationship between activating event as the independent variable and stress as the dependent variable. Data were collected from a sample of 201 teachers from seven Malaysian Fully Residential School (FRS) in the Johor state by using stratified random sampling. The Teacher Irrational Beliefs (TIB), Teacher Activating Event (TAE), and Teacher Stress (TRS) questionnaires were employed to measure irrational beliefs, activating events, and stress of teachers. The Pearson coefficient correlation was used to determine the relationships among variables and multiple regression analysis was used to verify the presence of mediation effects. In general, the correlation results showed that there were positive relationships among variables. The findings of regression analysis indicated that irrational beliefs mediate the relationship between activating event and stress among FRS teachers. This findings highlighted the teachers’ irrational beliefs as the major determinants of emotional problems rather than activating event itself which comply with the ABC Model based on Rational-Emotive Behavioral Therapy (REBT) approach.