Acicular magnetite (Fe3O4) powders were synthesized through new glycothermal dehydration by using crystalline alpha-FeOOH as precursor and glycols as solvent. When ethylene glycol was used as solvent, the phase was in-situ transformed from acicular alpha-FeOOH to alpha-Fe2O3 and finally to Fe3O4 at 270 degrees C for 6 h without morphological change. When water was added as a co-solvent in glycothermal reaction, Fe3O4 powders were synthesized through dissolution-recrystallization process at 230 degrees C for 3 h. The volume ratio of ethylene glycol to water (E/W) in the reaction has a strong effect on the morphology of the synthesized Fe3O4 particles. The particle shape of Fe3O4 particles changed from needle to sphere when the water content in E/W volume ratio increased from 0.5 to 1 mL in mixed glycothermal condition. When the water were added by more than 10 ml, the particle shape of Fe3O4 changed from sphere to octahedron truncated with the {100} faces and finally distinct octahedron with only {111} faces. Also, it is demonstrated that the size of Fe3O4 particles can be controlled from 1-2 mu m to 100-200 nm by varying the reaction conditions such as the volume ratio of water to ethylene glycol and additive in glycothermal reaction.