A simple and inexpensive approach to synthesizing mesoporous Fe3O4 is developed by using citric acid-assisted solid thermal decomposition of ferric nitrate. The structure and magnetic property of mesoporous Fe3O4 were characterized by XRD, FT-IR, N-2 adsorption-desorption isotherms, TEM, and vibrating sample magnetometer. It was shown that the decomposition of citric acid results in the formation of the mesoporous structure and narrow pore-size distribution. The reducing atmosphere which created by the decomposition of the ferric nitrate-citric acid complex caused the partial reduction of Fe(III) to Fe(II) and in turn the formation of Fe3O4. Moreover, the strength of the coordination between carboxyl group and Fe3+ also affected the phase composition of the iron oxides.