Purpose: The aim of this work was to assess the effect of (HBOT) on frozen shoulder in diabetic elderly. Patients and Methods: Sixty patients of both sexes represent the sample of this study. The patients' ages ranged from 66-75 years with a mean value of 67.90± 1.71 years. The patients were selected from Out patients’ Clinic at Air Force Hospital. They were assigned randomly into two groups equal in number; the study group (GA) and the control group (GB). (GA): treated by traditional medical treatment prescribed by treating physician and traditional physical therapy in the form of ROM exercise and Mobilization exercises 3 times per week for 6 weeks in addition to HBOT therapy. The Control group (GB): treated by traditional medical treatment prescribed by treating physician and traditional physical therapy in the form of ROM exercises and Mobilization exercises 3 times per week for 6 weeks. The duration of treatment was 6 weeks, HBOT therapy session was between 2-2.5 bar for 1 hour 5 times per week for 6 weeks (30 session) for group (A) only. Digital Inclinometer was used to measure shoulder joint degree, MRI for examine the shoulder joint to detect thickening of the Coracohumeral Ligament. Shoulder pain and disability index (SPADI) is a self-administered questionnaire that consists of two dimensions, one for pain and the other for functional activities assess before and after treatment in both groups. Results: Comparison of each variable pre and post treatment in each group revealed a significant improvement in all different parameters in study groups (GA) P ≤ 0.05; except for thickening of the Coracohumeral Ligament was not significant at both groups. Comparison of post treatment results of both groups showed that hyperbaric oxygen therapy used in study group (A) showed significant increase in ROM than group (B) and significant decrease in SPADI. Conclusion: Hyperbaric oxygen therapy have significant positive effect on frozen shoulder in diabetic elderly patients.