Mustapha, Mulikat Ladi AbdulqadirMUHAMMAD, Shuaib AbolakaleAlwajud-Adewusi, Mariam Bola2020-02-192020-02-192016-0313. Mustapha, M. L. A., Muhammed, S. A. & Alwajud-Adewusi, M. B. (2016): Effectiveness of Rational-Emotive-Behaviour therapy in reducing bullying behaviour among in-school adolescents based on gender and age. Journal of Science, Technology, Mathematics and Education (JOSTMED)12 (1); 284-293, Published by Department of Science Education, Federal University of Technology, Minna, Nigeria. https://jostmed.futminna.edu.ng/index.php/archives/item/20-jostmed-12-1-march-20160748-4710http://hdl.handle.net/123456789/3692Bullying behaviour is a deliberate, unprovoked, aggressive behaviour displayed repeatedly with the aim of harming or hurting others. It is characterised with imbalance of power (physical/psychological). This study investigated the effectiveness of Rational-Emotive Behaviour Therapy (REBT) in the reduction of in-school adolescents bullying behaviour. REBT is a theory based on the assumption that human beings are born with the ability of rational thinking and constructive behaviour. The theory attributes bullying behaviour to the irrational aspects of in-school adolescents belief system formed in childhood. The research design adopted in the study is quasi experimental in nature using 2×2×2 factorial design. It is made up of two (2) row groups (one experimental and one control) and two column groups (age and gender). Instrument used in purposively selecting the participants in the study was Bullying Identification Questionnaire (BIQ). Perpetrators of bullying behaviour (i.e bullies only) were identified and participated in the study. REBT treatment procedure packages were administered to participants in the treatment group. It was revealed that REBT produced significant reduction in the bullying behaviour of in-school adolescents, mean difference was observed in the pre and post-test scores of participants in the treatment group. ANCOVA was used to test for significant reduction of bullying behaviour yielding F= 85.8, df (1, 47); gender and age had moderating effects on the reduction of bullying behaviour of in-school adolescents F (df=1,47)=34.2; F (df=1,47) =51.9, p<0.05 respectively. It is recommended that REBT be used in the treatment of bullying behaviour of secondary school studentsenBullying behaviourIn-school adolescentsRational-emotive behaviour therapyArticle