The human cathelicidin LL-37 is a multifunctional host defense molecule that mediate various host responses, including antimicrobial action, chemotaxis, epithelial cell activation, angiogenesis, epithelial wound repair and activation of chemokine secretion. 1 2 3 4 5 6 However, LL-37 encounters the generic problems common to most linear counter parts such as short biological half-life, primarily arising from proteolytic susceptibility. A minimalized bacteriolytic domain of LL-37, referred to as KR-12 , has selective toxicity toward bacteria. 7 We investigated the strengths of the concepts of cyclization, disulfide bond formation and dimerization to improve both peptide stability and potency. Alanine/Lysine scan mutagenesis highlighted the residues critical for antimicrobial activity in KR 12, enabling us to design several double/triple mutants of KR-12. The backbone cyclized KR-12 dimers displayed a roughly 100-fold increase in permeabilization of E. coli-membrane vesicles as compared to its monomer. This translated into improved antibacterial, antifungal and antitumor cell activity, in particular one of the cyclic dimers displayed 10 fold better MIC against S. aureus and P. aeuroginosa than KR-12. Both KR-12 and retro KR-12 adopt helical structure in membrane environment. However, NMR and CD analysis reveal the cyclic dimers are unstructured, presumably because backbone cyclization restricts conformational freedom. Given the improved stability of the cyclic dimers in serum, backbone cyclization, disulfide bond formation and dimerization are useful tools in the development of stable antimicrobial peptide leads.