The impact of two structural factors of sodium carboxylate additives on their ability to induce the selective crystallization of gamma-aminobutyric acid (GABA) Form-II was investigated, namely, the number of -COO- groups and the types of R groups (chain alkyl, cyclic alkyl, or phenyl). It was found that the increase of -COO- groups caused the additives lose their inducing ability. This may be attributed to the fact that the increment of hydrophilic -COO- groups results in enhanced solvation and steric effect of additives, which makes additives unable to interact with GABA effectively. Additives with different R groups all can induce Form-II but they exhibited varying inducing abilities. This could be due to the distinct results of competition between the attachment and detachment of different additives on the GABA growth end.