One of the main bases in Firdawsi’ s contemplations and one of the mostly applied words and concepts in Shahnameh (Firdawsi’ s book) is “ justice (Daad), so that some scholars have called Shahnameh as the “ Epic of Justice” . In general, according to Firdawsi, justice so flows in nature and life that he considers death, which is the most complicated and mysterious issues, as a justice in the world. Firdawsi sees a fundamental relationship between justice and happiness, peace, security, development, and on the other hand between injustice and dissatisfaction, restlessness, and destruction. Although he considers justice as everyone's duty, but there is a prominent connection between justice and kingdom in his book, so that justice is the most eminent characteristic of the kings in Shahnameh, especially at the beginning of their reign when they express their manifest. This article uses a descriptive analytic method in dealing with the relationship between justice and kingdom in Shahnameh. One of the results in this study indicates the deviation in the criteria for justice and injustice in Shahnameh at different periods. Justice is sometimes measured by religious criterion. However, the criterion for justice at different eras is mainly the kings themselves, since they psychologically consider themselves to be right. The concept of justice in Shahnameh (specifically based on contemporary standards, especially in mythical part of Shahnameh) has a paradoxical concept full of challenges.