The aim of the present study was to determine the effectiveness of cognitive rehabilitation on working memory and processing speed. The present study was a quasi-experimental study with a pre-test-post-test design with a control group. Among children aged 7-12 years diagnosed with attention deficit/hyperactivity disorder based on DSM-5 criteria, 30 children were selected from Tabriz psychiatric centers in 2018 and randomly divided into two experimental and control groups. Demographic information questionnaire and Wechsler IV working memory and processing speed tests (subtests of numerical capacity, number-letter sequence, symbol recognition, and coding) were the instruments of this study, and based on the results of the pre-test, the experimental group participated in 11 94-minute cognitive rehabilitation sessions. Also, after the intervention, the children were re-evaluated. The data were analyzed using SPSS18 software using descriptive analysis and MANUFACTURING tests. The results of the analysis of covariance showed that the working memory score (number capacity) and processing speed (symbolization and encoding) of the experimental group in the post-test were significantly different from the control group (p<0.001), but no significant difference was observed between the two groups in the number-letter sequence test. Based on the findings of this study, it can be said that cognitive rehabilitation has an effect on processing speed and some of the working memory symptoms of children with attention deficit/hyperactivity disorder.