A tentative theory is presented in which β-phase PdDx containing supersaturated deuterium transits into β-phase PdDx containing less deuterium and α-phase PdDx. High-pressure (∼10 GPa) deuterium bubbles form at the same time. As the bubbles release energy, cracks are created in the PdDx crystal, and charge separation of deuterium occurs. Thus would cold fusion be induced. This proposal supports the fracture mechanism for cold fusion.