This　review　summarizes　our　studies　in　the　development　of　small　cyclic　peptides　for　specifically　modulating　enzyme　activity.　Serine　proteases　share　highly　similar　active　sites　but　perform　diverse　physiological　and　pathological　functions.　From　a　phage-display　peptide　library,　we　isolated　two　mono-cyclic　peptides,　upain-1　(CSWRGLENHRMC)　and　mupain-1　(CPAYSRYLDC),　which　inhibit　the　activity　of　human　and　murine　urokinase-type　plasminogen　activators　(huPA　and　muPA)　with　Ki　values　in　the　micromolar　or　sub-micromolar　range,　respectively.　The　following　affinity　maturations　significantly　enhanced　the　potencies　of　the　two　peptides,　10-fold　and　＞　250-fold　for　upain-1　and　mupain-1,　respectively.　The　most　potent　muPA　inhibitor　has　a　potency　(K-i　=　2　nM)　and　specificity　comparable　to　mono-clonal　antibodies.　Furthermore,　we　also　found　an　unusual　feature　of　mupain-1　that　its　inhibitory　potency　can　be　enhanced　by　increasing　the　flexibility,　which　challenges　the　traditional　viewpoint　that　higher　rigidity　leading　to　higher　affinity.　Moreover,　by　changing　a　few　key　residues,　we　converted　mupain-1　from　a　uPA　inhibitor　to　inhibitors　of　other　serine　proteases,　including　plasma　kallikrein　(PK)　and　coagulation　factor　XIa　(fXIa).　PK　and　fXIa　inhibitors　showed　Ki　values　in　the　low　nanomolar　range　and　high　specificity.　Our　studies　demonstrate　the　versatility　of　small　cyclic　peptides　to　engineer　inhibitory　potency　against　serine　proteases　and　to　provide　a　new　strategy　for　generating　peptide　inhibitors　of　serine　proteases.