Background & objectives: Dysthimia is a chronic, therapy-resistent disorder recongnized through a depressive mood in most days and hours. Dialectical behavior therapy is a practical treatment approachin for major depressive disorder. Dysthimia symptoms are like major depression but with longer duration and lower severity. So it is assumed that dialectical behavior therapy may be effective in dysthimia treatment. So the present study was done with the aim of determining the effectiveness of dialectical behavior therapy in treatment of dysthymia disorder.

Methods: This research was an experimental study and used a pre-test/post-test design with control group. The study population were all the men and women of 20 to 50 years old who resided in Karaj in the year 2013 and had visited clinics and mental health centers with diagnosis of dysthymic disorder. Convenience sampling method was used to choose 75 people which randomly allocated to treatment and control groups. This sample decreased to 20 in each group because of drop out.  Rosenberg's self-esteem scale, Beck's Depression inventory and DASS depression, anxiety, and stress scale were used for data gathering. The data were analyzed by multi-variant ANOVA using SPSS (v.16).

Results: We found a significant difference between mean (F=3.337) and the total score of depression (F=3.95) between two groups. According to the data, dialectical behavior therapy led to treatment of depression.

Conclusion: According to the findings dialectical behavior therapy is effective in treatment of the signs and symptoms of dysthymia and it can be used along with medicine and other cognitive therapies.