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Aim and Background: Today, diabetes is one of the most common chronic diseases. Previous researches indicate the effect of lifestyle on the increased incidence of the disease. The purpose of the study was to assess the efficiency of lifestyle modification training on psychological symptoms and glycemic control among patients with type II diabetes. Methods and Materials: This was an experimental clinical trial with a pretest-posttest design, control group, and follow-up period. The statistical population included all patients with type II diabetes referred to the Glands and Metabolism Research Center in the Medical Science University of Isfahan, Iran, in 2013. Accordingly, a sample of 42 diabetics was selected using convenience sampling and they were randomly allocated to experimental (n = 21) and control (n = 21) groups. The Symptom Checklist-90-R (SCL-90-R) and blood test for HbA1c were utilized in pre-test, post-test, and 3 months after the intervention (follow-up). The study was implemented during 6 months. The interventions were performed in the experimental group within 8 weekly sessions lasting 90 minutes for 2 months. The data were analyzed using MANCOVA in SPSS software. Findings: Lifestyle modification training had significant effects on psychological symptoms (on physical complaints, obsession-compulsion, interpersonal obsessive disorder (IOD), depression, anxiety, aggression, paranoid, and melancholia, except phobia), blood sugar levels of patients with type II diabetes at post-test and follow-up stages (P < 0.05). Conclusions: According to the results, it can be concluded that lifestyle modification training is effective on the reduction of mean psychological symptoms scores and blood sugar levels in patients with type II diabetes. In addition, planning and holding educational courses according to the educational package of the present study is effective on the improvement of these patients. Thus, it is suggested that the efficacy of this intervention be evaluated in other chronic illnesses. Moreover, the results of this study can be exploited by doctors, nutritionists, counselors, and psychologists.