Flavodoxins, which are wildly existent in microorganisms，have been found in various pathways with multiple physiological functions. The flavodoxin containing the cofactor flavin mononucleotide (FMN) from sulfur reducing bacteria Desulfovibrio gigas is a short-chain enzyme comprising 146 amino acids with a molecular weight of 15 kD and plays important roles in the electron transfer chain. In present work, we have purified the flavodoxin directly from massive anaerobically grown Desulfovibrio gigas cells for crystallization and structure determination. The crystallization was performed by the hanging-drop vapor-diffixsion method at 18°C，and the crystal qualities were further improved by the micro-seeding technique. According to diffraction data at 1.3 A resolution, the crystals of flavodoxin belong to the orthorhombic space group P212121 with unit-cell parameters a = 50.20 A， b = 60.37 A and c = 76.25 A. The analysis of the Mathew's coefficient and the self-rotation function indicate that there are two flavodoxin molecules in an asymmetric unit with the solvent content of 40.4%. The preliminary structure solution showed that the structure of flavodoxin is a dimer with the head-to-head orientation.