The purpose of this study was to investigate whether training of writing a daily work log using cognitive therapy approach can help increase work performance in a patient with depression using a single subject experimental reversal design (ABA). Method : The patient is a 19-year male youth with a diagnosis of depression of which onset was 2 years. While being on a work-aid program for individuals with various disabilities, he has shown helplessness, inferiority with his appearance, insomnia, and suicidal impulse besides depression. His supervisor reports that he often fails to complete his own work, makes errors and mistakes, and cuts his work hours. His training of writing a daily work log consisted of the following steps; (a) analyzing and writing about the circumstances, his behaviors, thoughts, and feelings at the happiest and unhappiest work or time on a daily basis; (b) analyzing and writing his plans about how to improve his unhappy feelings or circumstances, implement them accordingly, and the results post implementation of these plans; (c) reviewing his daily work log and discussing about it with the main investigator daily; and (d) writing 3 times and reading aloud 2 times his resolutions regarding the unpleasant circumstances, methods to improve the circumstances or feelings, and the expected positive results from his resolutions. This training was conducted for 5 days. Results : The results were: on self-reporting depression scale, a mean of 80% in the baseline and 68% in the training; on self-reporting on the patient’s work performance satisfaction, a mean of 68% in the baseline, and 88% in the training; the supervisor’s reporting of the patient on the Work Personality Profile, a mean of 58% in the baseline and 70% in the training; and on the self-reporting on the Dysfunctional Attitude Scale, 215 out of 280 full points in the baseline and 164 in the training. Conclusions : The results indicate that the use of writing a daily log using cognitive approach helps improve not only depression state but also work performance in a patient with depression.