We explore a miniature sensor node that could be placed in an environment which would interrogate, take decisions and transmit autonomously and seamlessly without the need of a battery. With the system completely powered by an energy harvester for autonomous operation, the power management becomes crucial. In this paper, we propose an ultra low power management circuit implemented in 0.18µm CMOS technology. As part of a stringent power requirements and very limited power offered by the energy harvesters, the proposed circuit provides a nanowatt power management scheme. Using post-layout simulation, we have evaluated the power consumption of the proposed power management unit (PMU) and report results that compares favorably to the state of the art.