A wearable energy harvester technology is developed for generating electricity from the movement of human joints. A micro-electroplated ferromagnetic nickel cantilever is integrated with a piezoelectric element and bonded on a flexible substrate. Based on the magnetic interaction between the magnetized cantilever and a magnet on the substrate, a novel vertical-vibration frequency-up-conversion (FUC) structure is formed to generate stable amounts of electric energy per cycle from the horizontal substrate stretching/rebounding. The two ends of the flexible substrate are attached on both sides of a limb joint to transform joint rotation into substrate stretching. During limb movement, the flexible substrate is horizontally stretched and rebounded, causing the cantilever to vertically release from and return to the magnet, thereby exciting the piezoelectric cantilever into resonant generation. Since the horizontal low-frequency limb movement is perpendicular to the vertical high-frequency resonance, the stretch has little influence on the resonance of the cantilever. Thus the generated energy is always stable within a wide frequency range of limb movements. The performance of the novel harvester is experimentally verified using a stretching/rebounding movement cycle, where the cycle corresponds to the frequency range of 0.5-5.0 Hz. Within one stretching/rebounding movement cycle, the generated electric energy is stable in the approximate range of 0.56-0.69 μJ for the whole frequency range. Two flexible harvesters are worn on the human elbow and knee for a body kinetic energy harvesting test. Considerable power can always be generated under typical low-frequency limb movements, such as squatting, walking, jogging, and fast running, where the peak-to-peak generated voltages are always approximately 4.0 V. Additionally, energy harvesting under two-directional area stretching is also realized by adjusting the FUC structure layout. The flexible-substrate harvester is promising for various wearable applications.