Software defined networking (SDN) is currently regarded as one of the most promising paradigms of future Internet. Although the availability and scalability that a single and centralized controller suffers from could be alleviated by using multiple controllers, there lacks a flexible mechanism to balance load among controllers. This paper proposes a load balancing mechanism based on a load informing strategy for multiple distributed controllers. With the mechanism, a controller can make load balancing decision locally as rapidly as possible. Experiments based on floodlight show that our mechanism can balance the load of each controller dynamically and reduce the time of load balancing.