The quaternization of long chain alkyl tertiary amine with dimethyl carbonate was studied. The effects of reaction temperature, reaction time, material mol ratio and solvent dosage on quaternization of N, N-dimethyldodecylamine were examined, and the optimized reaction condition was determined. The optimized condition was : reaction temperature 130 ℃, reaction time 5 h, n (dimetyl carbonate)︰n (alkyl tertiary amine)=5, methanol dosage (based on the materials) 10%. Under the optimized conditions, the conversions of mono-alkyl tertiary amines with different carbon chain length could reach over 98%, while the conversions of N-methyldidecadeamine and N-methyldioctadecylamine were 88.09% and 68.77% respectively. These indicated that space steric hindrance was the main factor for the quaternization process, consisting with the results of molecular space structure simulation. Through extending reaction time, the conversion of N-methyldidecadeamine can reach to 98.96% after 7 h and N-methyldioctadecylamine can reach to 95.68% after 12 h. The quaternary ammonium salts were characterized by 1H-NMR and IR. The results showed that quaternary methyl carbonates were synthesized successfully.