Abstract: There is a growing interest in the study of structures and properties of biomolecules in gas phase. Applications of force fields are highly desirable for the computational efficiency of the gas phase study. To help the selection of force fields, the performances of five repre-sentative force fields for gaseous neutral, protonated, deprotonated and capped amino acids are systematically examined and compared. The tested properties include relative conforma-tional energies, energy differences between cis and trans structures, the number and strength of predicted hydrogen bonds, and the quality of the optimized structures. The results of BHandHLYP/6-311++G(d,p) are used as the references. GROMOS53A6 and ENCADS are found to perform poorly for gaseous biomolecules, while the performance of AMBER99SB, CHARMM27 and OPLSAA/L are comparable when applicable. Considering the general availability of the force field parameters, CHARMM27 is the most recommended, followed by OPLSAA/L, for the study of biomolecules in gas phase.