For applications of the microwave power transmission technology, it is necessary to realize high efficiency rectennas or rectification circuits. Especially, RF-ID's or ubiquitous power sources demand high RF-DC conversion efficiency for below mW microwave inputs. However, almost all past rectennas or rectification circuits were designed for over 100mW rectifying and the RF-DC conversion efficiencies were less than 20% at 1mW microwave input. In this paper, we investigate rectification circuits for developing mW-class rectennas. Discussing diode mount structures in order to increase the efficiency below 10mW microwave input, we succeeded in developing a rectification circuit whose efficiency is 62.8% for a 10mW microwave input. We also find that approximately 30% of the input microwave power is consumed by the diode in the rectification circuit.