Abstract: ¹¹C methylation was very important methods for synthesis of PET radiopharmaceuticals. It included C-N, C-O, C-S methylation and so on. There are many influence factors for ¹¹C methylation, such as methylation agents, the base of reagents, the polarity of solvent, the obstruct of space. We can adjust the methylation agent or change solvent in order to make one reaction as main reaction when a compound had two activity atoms.