Abstract: N succinimidyl 4 18 F (fluoromethyl)benzoate ( 18 F SFMB) of labeling proteins has been synthesized. Laborious and time consuming HPLC is substituted by simple Sep pak silica during the process of isolation. Many factors affecting the radiochemical synthesis of 18 F SFMB including reaction solvents, temperature, the mole ratio of K/222 to K 2CO 3 and reaction time are studied. The better labeling conditions are as follows:dry acetonitrile as solvent, the mole ratio of K/222 to K 2CO 3 is 1∶1, reaction time is 5 min at 80 ℃. The radiochemical yield is about 57%(EOS) which is about 3 times higher than the published data. IgG is indirectly labeled by 18 F SFMB and the labeled yield are about 88% at room temperature.