This study was to assess the inhibitory effects of theanine (T) and TClC (a synthesized novel theanine derivative) on the growth of highly-metastasis human breast MDA-MB-231 cells and explore the mechanisms of such an inhibition. The MTT assay was used to evaluate the effects of different concentrations of T and TClC on the grown of in vitro MDA-MB-231 cells. Western blot analysis was employed to detect the expressions of apoptosis-related proteins and the possible targets of drug actions in the MDA-MB-231 cells. The experimental results showed that the inhibitory effects of T and TClC on the growth of MDA-MB-231 cells are gradually enhanced with the increase of the concentrations. TClC showed a much stronger inhibition of cell growth than T. T. TClC down-regulated the expression level of antiapoptotic Bcl-2 protein and up-regulated the expressions of proapoptotic Bax protein, leading to the reduction of Bcl-2/Bax ratio. In addition, T and TClC reduced the protein level of NF-κB and VEGFR1. TClC exhibited much stronger effects on regulating these proteins than T. The expression regulation of these proteins by TClC and T might be one of the important mechanisms of the cell growth suppression. All these findings suggested that T and TClC may have potential to the treatment of highly-metastasis human breast cancer.