Background: Despite the significant clinical benefit in hemoglobin A1c reduction, diabetes applications (apps) use is low. The goal of this study was to assess app usability based on the heuristics of intuitive designs and the Theory of Self-Determination on motivation. Diabetes apps were evaluated by clinicians and patients for motivational properties that target patient needs to learn blood glucose trends and patterns (competence), to personalize choices to change (autonomy) and to connect with healthcare providers (feel related). Method: Two top-rated Android apps, mySugr and OnTrack, underwent evaluation by four clinicians (three physicians and one diabetes nurse educator) and 92 adults with type 1 or 2 diabetes on insulin. Each app was tested for seven diabetes-related tasks including data entry, analysis, report view options (by the day of the week and by meals), and report email. Clinicians evaluated if the app violated Nielsen's Ten Heuristics on user-friendly designs. Both clinicians and patients completed a questionnaire on the ease of use - System Usability Scale (SUS). Results: OnTrack and mySugr target two behavior needs important for motivation in diabetes self-management. App usability was suboptimal to low. Clinicians rated an “F” for mySugr (15 ± 5) scoring < 60 and a “D” for OnTrack (61 ± 28) scoring between 60-69. Patients rated an “F” for mySugr (55 ±18) and a “D” for OnTrack (68 ±15). Clinicians identified many heuristic violations (n=169): mySugr (90) and OnTrack (77). Heuristic principles violated the most frequently were “Help and Documentation” (38), followed by “Aesthetic and Minimalist Design” (43) and “Error Prevention” (24). Conclusions: Commercially available diabetes app usability was marginally acceptable to unacceptable according to clinicians and patients. Future diabetes app design needs to target patient motivation and incorporate key heuristic principles to provide self-tutorials, enhance graphical or screen views, and eliminate error-prone conditions. Disclosure H.N. Fu: None. T. Adam: Stock/Shareholder; Self; GlaxoSmithKline plc. J.F. Wyman: None. Funding Robert Wood Johnson Foundation; Sigma Theta Tau International-Zeta Chapter