The purpose of this study was to investigate the mechanism of the effect of behavioral activation therapy (BA) through cognitive flexibility and emotional flexibility on the symptoms of women with major depressive disorder. This study was based on the single-subject design. The research sample consisted of five women with major depressive disorder (BDI-II), Beck Anxiety (BAI), Cognitive Flexibility (CFI) and Emotional Flexibility (EFI) at baseline and during the stage of depression. Treatment was evaluated. To analyze the data, trend, slope, and charts were used to determine the clinical significance of the percentage of recovery, percent reduction, effect size and Cohen index. The findings indicated that behavioral activation therapy significantly contributed to enhancing the cognitive flexibility and emotional flexibility of patients with major depressive disorder. In general, the results of the research indicated that the benefits of behavioral activation treatment as an effective intervention were used to increase the cognitive flexibility and emotional flexibility of women with major depressive disorder.