Wireless Sensor Network (WSN) has been used in widespread domains, and the real-time response is required by many WSN applications. However, due to the memory resources limitation on the sensor nodes, the current WSN OSs such as TinyOS, Contiki, SOS, mantisOS, etc., are not real-time ones. To achieve the objective of designing a realtime OS with low memory resource consumption, a new WSN OS named HEROS is developed and presented in this paper. For HEROS, it adopts a hybrid scheduling strategy. Both the event-driven and multithreading schedulers are implemented in parallel, and these two schedulers can switch to each other when necessary.