Vacancy generation process m nearly perfect aluminum single crystal was investigated by means of a shape of helical dislocation by X-ray diffraction topography, using a translation type high temperature Lang camera. Helical dislocations as a vacancy source were observed after temperature rise . The shape of helix at initial stage of the appearance looked like a coil spring. But the shape changed to the helix refiected the crystal structure of specimen with the lapse of time. Under-saturation of vacancy in the specimen was estimated from the size of helix. In the results , most of vacancies were emitted from dislocations existed in as-grown nearly perfect crystals, and it takes several hours to achieve the equilibrium concentration of vacancy after a temperature rise.