With low-power consumption, small code and data size, evolvability as design criteria, we develop an evolvable operating system (EOS) for wireless sensor network applications. The EOS provides memory space efficient thread management, collaborative thread communication model and network stack. It also supports power management of microcontroller and radio transceiver, and network wide time synchronization function. Above all, the most important feature is the concept of evolvability with which the operating system itself can be easily configurable and upgradeable.