Purpose This study aims to investigate the effect of the Diamond Justice model on self-efficiency with the mediating role of job stress among the staff of Qazvin hospitals affiliated with Qazvin University of Medical Sciences. This study is a cross-sectional descriptive-analytical study conducted among the staff of Qazvin hospitals affiliated with Qazvin University of Medical Sciences in 2020. Design/methodology/approach Sampling was performed using the structural equation method. Data collection tools included three sections: demographic information, justice and self-efficiency questionnaire and job stress questionnaire. Data were finally analyzed using SPSS software version 26 and AMOS version 23 at a significance level of 0.05. Findings The structural equation model’s standard estimation coefficients show that all existing paths are at a significant level. Finally, the regression analysis showed that justice is inversely related to stress level (ß = −0.185, p = 0.015). Justice is directly related with self-efficiency (ß = 0.282, p < 0.001). Originality/value Justice, stress and self-efficacy have been measured in various studies among health workers. However, a fitting model showing these three variables’ interaction was necessary. Therefore, this study tries to conceptualize the multifaceted relationships of the components of these concepts by presenting a model.