Sub-micrometer diamond tube arrays are formed on freestanding diamond film via focused ion beam pattern technology and chemical etching method. First, the sub-micrometer holes are fabricated on Si substrate by using FIB milling method. Then, diamond film is grown by hot filament chemical vapor deposition method on patterned Si substrate. By controlling the deposition parameters, the diamond can be grown along the wall of holes and the diamond tubes in sub-micrometer scale are formed. Finally, Si substrate is etched by chemical etching method and the diamond tube arrays are fabricated on a freestanding diamond film. Scanning electron microscopy and Micro-Raman spectroscopy measurements are performed to characterize the structure and phase purity of diamond tubes. The electron emission properties from the diamond tube arrays are studied, the result presents that the enhanced emission property can be obtained from diamond tube arrays.