This paper presents a 0.6 V input, 0.3-0.55 V output buck converter in 40 nm CMOS, for low-voltage low-power wireless sensor network systems. A low power CCM/DCM controller of the buck converter enables automatic selection of DCM or CCM operation depending on load situation, therefore improving the power efficiency. A dual-mode-body-biased (DMBB) zero-crossing detector with both forward body bias mode and zero body bias mode is designed to enable DCM operation with both low supply voltage and normal supply voltage. An ultra-low-power hysteresis voltage detector is proposed for body bias modes selection. The proposed buck converter achieves a peak efficiency of 94% with an output current range of 50 μA to 10 mA. Thanks to the DCM operation, the efficiency at an output current of 10 μA is improved by 20% and 9%, with an output voltage of 0.35 V and 0.5 V, respectively.