Two kinds of Kunitz-type protease inhibitors, AKPI1 and AKPI2, were purified from Apios americana tubers by four steps of column chromatographies and their cDNA cloning was performed. AKPI1 cDNA consist of 809 nucleotides, and the matured protein had 190 amino acids with 20,594 Da. AKPI2 cDNA consist of 794 nucleotides, and the matured protein had 177 amino acids with 19,336 Da. P1 site of AKPI2 was Leu88, suggested the target enzyme was chymotrypsin. On the other hand, Gly85-Ile86-Ser87 was positioned around P1 site of AKTI1. Sequence analysis suggested that two forms (single-chain and two-chain form) of AKPI2 protein were present in the tubers. Recombinant AKPI2 expressed by E.coli system showed inhibitory activity toward serine proteases and heat stability. The Ki values toward chymotrypsin and trypsin were 4 × 10−7 M and 6 × 10−6 M, respectively. Abbreviations: AAL: Apios americana lectin; AATI: Apios americana Bowman-Birk type trypsin inhibitor; ACE: angiotensin-converting enzyme; IPTG: isopropyl-β-D-thio-galactopyranoside; Ki: inhibition constant; KPIs: Kunitz-type protease inhibitors; L-BAPA: Benzoyl-L-arginine p-nitroanilide monohydrochloride; L-BTPA: Benzoyl-L-tyrosine p-nitroanilide; PFLNA: Pyr-Phe-Leu-p-nitroanilide; RP-HPLC: reverse-phase high-performance liquid chromatography; RT-PCR: reverse transcription-polymerase chain reaction; SDS-PAGE: sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SLIC: sequence and ligation independent cloning; STANA: N-Succinyl-Ala-Ala-Ala-p-nitroanilide; SHR: spontaneously hypertensive rats; TFA: trifluoroacetic acid; UTR: untranslated region.