Plant　peptidase　inhibitors　play　essential　roles　in　the　defense　systems　of　plants.　A　trypsin　inhibitor　(PHTI)　with　a　molecular　mass　of　20.5　kDa　was　isolated　from　the　fresh　roots　of　the　medicinal　herb,　Pseudostellaria　heterophylla.　The　purification　process　involved　ammonium　sulfate　precipitation,　gel　filtration　chromatography　on　Sephadex　G50,　and　ion-exchange　chromatography　on　DEAE　650M.　The　PHTI　contained　3.7%　alpha-helix,　42.1%　beta-sheets,　21.2%　beta-turns,　and　33%　disordered　structures,　which　showed　similarity　with　several　Kunitz-type　trypsin　inhibitors.　Inhibition　kinetic　studies　indicated　that　PHTI　was　a　competitive　inhibitor,　with　a　Ki　value　of　3.01　x　10(-9)　M,　indicating　a　high　affinity　to　trypsin.　The　PHTI　exhibited　considerable　stability　over　a　broad　range　of　pH　(2-10)　and　temperatures　(20-70　degrees　C);　however,　metal　ions,　including　Fe3+,　Ba2+,　Mn2+,　and　Al3+,　could　inactivate　PHTI　to　different　degrees.　Results　of　fluorescence　spectroscopy　and　circular　dichroism　showed　that　Fe3+　could　bind　to　TI　with　an　association　constant　of　2.75　x　10(5)　M-1　to　form　a　1:1　complex,　inducing　conformation　changes　and　inactivation　of　PHTI.　In　addition,　PHTI　could　inhibit　the　growth　of　the　phytopathogens,　Colletotrichum　gloeosporioides　and　Fusarium　oxysporum,　through　disruption　of　the　cell　membrane　integrity.　The　present　study　extended　research　on　Pseudostellaria　heterophylla　proteins　and　makes　PHTI　an　exploitable　candidate　as　an　antifungal　protein　for　further　investigation.