Solid wastes of freshwater clam in food processing, including mainly mantle, were used as a raw material for the recovery of bioactive peptides related to angiotensin I-converting enzyme (ACE) inhibitor. Among the primary hydrolysates of dried mantle (DM), the peptides corresponding to hydrolysates using 2 crude peptidases exhibited a strong ACE inhibitory activity ($IC_{50}$, 0.23 mg/mL), and recovery efficiency of soluble materials and their protein content were considerably large with 42.65% and 468.6 mg/g, respectively. The ACE inhibitory activity of all secondary hydrolysates digested by pepsin and trypsin was significantly increased as compared to primary hydrolysates. Furthermore, the peptic secondary hydrolysates were fractionated by gel filtration and reverse phase-HPLC (RP-HPLC) and characterized by matrix-assisted laser desorption-ionization time of flight/mass spectrometry (MALDI-TOF/MS). These peptides with molecular weight of less than 1 kDa possessed the stronger ACE inhibitory effect, and their inhibitory pattern was found to be competitive. The results showed that the DM hydoplysates might be utilized as a rich source of bioactive peptide.