Wireless sensor nodes self-powering from an ambient energy source have been widely studied. In particular, the piezoelectric energy harvesting has been the focus of attention, due to its high energy density. For the piezoelectric energy harvesting, a cantilever beam structure has normally been used because of its high efficiency. Since the frequency of environmental vibrations is of the order of hundreds of Hz, which is far below the natural frequency of cantilever type ceramic actuators, the frequency of the cantilever has commonly been adjusted by placing a heavy mass on its end. In order to lower its natural frequency and increase its safety factor, we fabricated a flexible piezoelectric cantilever, which was composed of ceramic on a flexible substrate. However, it was observed that the flexible cantilever was always broken at a certain position. This result indicates that the stress was not homogeneous, and concentrated at the breakage point of the ceramic. To analyze the failure of the ceramic, we measured the inhomogeneous stress distribution on the flexible piezoelectric energy harvester and analyzed it using the ATILA program.