Energy harvesting is a key technology in various small-size applications such as wireless sensor nodes, mobile devices, and implantable bio-devices to improve battery lifetime or to substitute for batteries. Piezoelectric (PE) transducers are popular energy harvesters that can be used to supply AC power at the μW to mW scale to electronic devices using ambient vibrational energy. To use PE energy effectively, the harvesting systems need a highly efficient AC-DC converter and a DC-DC converter with maximum power point tracking (MPPT). However, existing sub-mW converters do not include an integrated MPPT algorithm [1, 2] or use the perturb and observe MPPT method which has long tracking time [3, 4]. This long tracking time reduces the power extraction from the transducer because the maximum power point (MPP) of the input power can be changed frequently according to the environment. In this paper, a low-power harvesting system is presented that finds the MPP of the input power in one cycle of the vibration of the PE transducer.