Interdependent networks, where two networks depend on each other, are becoming more and more significant in modern systems, e.g., the control and transmission networks in the smart grid. From previous work, it can be concluded that interdependent networks are more vulnerable than a single network. The robustness in interdependent networks deserves special attention. In this paper, we propose a metric of robustness from a new perspective — the balance. We define the balance-coefficient of the interdependent system. Some evaluations verify the impact of the balance property.