Abstract: The interaction of 2-(acetyl-ethoxycarbonyl )-methylene-tetrahydroimidazole (L )and Fe(Ⅲ) .Fe (Ⅱ),Cu (Ⅱ) were investigated by using fast atom bombardment mass spectrometry(FABMS) technique. Fe(Ⅲ) ion formed stable metal complex (FeL,+)when Fe(Ⅲ) was added in the ligand. However,Fe(Ⅲ) and Cu(Ⅲ) promote the polymerization of ligand, and the observation of different lower polymer ions in mass spectra indicated that, for the polymerization,Cu (Ⅱ) is more efficient than Fe (Ⅱ). Furthermore,in order to explain the experimental results,the possible competition mechanism between the coordination bond and hydrogen bond in the metal complex interrnediate has been proposed.