Additive is an effective factor in controlling crystallization of polymorph. L-Tryptophan, L-leucine, L-threonine, L-histidine, L-alanine, and L-valine, as additives during L-glutamic acid crystallization, were investigated for probing their effects on the polymorphic transformation. L-Tryptophan was then chosen as a representative case to evaluate the effect of addition concentration on the morphology, nucleation, and polymorphic transformation rate of L-glutamic acid. It is shown that all these additives have a certain influence on the polymorphic transformation and the presence of a bulky side chain (phenyl) in L-tryptophan and L-histidine is identified as an essential feature in achieving stabilization of the a-polymorph. When the concentration of L-tryptophan reaches above 0.008 M, the elongation of crystals occur. Increasing L-tryptophan concentration, the transformation time and nucleation rate of L-glutamic acid are retarded. The inclusion behavior of additive molecules in L-Glu crystals was investigated, and the results show that the effect of additive molecule on the polymorphic transformation may be through incorporating into crystal structures by adsorption.