The fractofusion mechanism of coldfusion is investigated theoretically. The conditions necessary for fractofusion during the absorption of deuterium atoms by palladium specimens (the condition of so-called coldfusion experiments) is clarified, including crack generation at grain boundaries, the high orientation angle of grains, rapid crack formation, the increase of electrical resistance around a crack, the large width of cracks, and the generation of many cracks. The origin and quantity of the electrical field inside cracks in the conductor are clarified also. By the fractofusion mechanism, the experimental facts that neutron emissions are observed in bursts, that sometimes they coincide with the deformation of a palladium specimen, and that in many experiments excess neutrons were not observed are qualitatively explained. The upper limit of the total fractofusion yields during the absorption of deuterium atoms by palladium specimens are estimated. The upper limit of the total yield increases to 7 × 106/cm3 palladium as the grain size increases to 1 mm. This value is larger than those observed in many experiments. It indicates that the experiments can be explained by the fractofusion mechanism.