The aim of the present study was to investigate the effectiveness of cognitive behavioral therapy intervention on promoting happiness and adaptive behaviors and reducing maladaptive behaviors in children with cancer aged 7 to 13 years in Urmia city. This study was conducted using a quasi-experimental pre-test-post-test method with a control group. The statistical population included all children with cancer aged 7 to 13 years who had referred to Urmia medical centers between January 2004 and June 2005. 40 children were selected through convenient and purposeful sampling and were randomly assigned to two experimental and control groups (20 people). The experimental group underwent cognitive behavioral therapy intervention for 8 60-minute sessions, and the control group received only usual medical care. The measurement tools were the Children with Cancer Happiness Scale (MSCS) and the Behavior Assessment Questionnaire (BASC-2). Data analysis was performed using the analysis of covariance (ANCOVA) test and SPSS software. The results showed that the experimental group had a significant increase in happiness scores (F=12.76; p<0.01; Eta²=0.35) and adaptive behavior (F=9.34; p<0.01; Eta²=0.29) compared to the control group, and a significant decrease in maladaptive behavior (F=10.25; p<0.01; Eta²=0.30). The findings of the study confirm the positive effect of cognitive behavioral therapy on improving happiness and behavior in children with cancer.