Background: Insulin-naive patients are often reluctant to receive insulin treatment, and even insulin-treated patients frequently have poor rates of adherence to their prescribed insulin injection regimes. Assessing attitudes toward insulin injection may help in the design of interventions that improve the insulin injection behaviors of patients with type 2 diabetes (T2DM). The concept of decisional balance has been associated with behavior in many studies and may be useful in assessing the attitude of patients with T2DMtoward insulin injection. Decisional balance for insulin injection (DBII) has not been widely assessed in patients with T2DM. Purpose: The aim of this study was to develop an insulin injection (DBII) scale that is valid for insulin-naive and insulin-treated patients and to test the psychometric characteristics of this scale based on the concept of decisional balance. Methods: This cross-sectional study administered an 18-item DBII scale, including pro and con subscales, to 95 insulin-naive and 237 insulin-treated patients in Taiwan. The decisional balance score was calculated as the mean score of the pro subscale minus the mean score of the con subscale. Construct validity was examined using exploratory factor analysis and confirmatory factor analysis; concurrent validity was assessed by examining the association between the score of the DBII scale and the stages of behavioral change and of hemoglobin A1c for, respectively, insulin-naive patients and insulin-treated patients; and reliability was assessed using internal consistency and test– retest reliability. Results: A 13-item DBII scale supported by exploratory factor analysis and confirmatory factor analysis was developed. The stages of behavioral change and hemoglobin A1c levels were found to be significantly associated with the scores of decisional balance of the 13-item DBII scale for both insulin-naive and insulin-treated patients. The Cronbach's α ranged between .78 and .92. Conclusions: The 13-item DBII scale is appropriately short and possesses satisfactory validity and reliability for both insulin-naive and insulin-treated patients with T2DM. Healthcare providers may use this scale as a checklist to guide clinical discussions related to insulin therapy with both insulin-naive and insulintreated patients with T2DM across time.