The present invention relates to novel linear and cyclic polypeptides that bind to IL-23 receptor and inhibit the binding of IL-23 to its corresponding receptor and cell signaling thereof. The novel polypeptides of the present invention has a core structure of WX1X2X3W, where W is tryptophan, and X1, X2 and X3 are amino acids, with the proviso that when one of X1, X2 or X3 is W, the remaining two of X1, X2 or X3 cannot be W. Preferred core structures include WVDYW or WQDYW. The present invention relates a composition containing the novel polypeptides (linear or cyclic), and use of same in inhibiting cell functions including production of IL-22 and IL-17F from immune cells as well as in treating IL-23 associated human diseases including, for example, inflammatory bowel diseases, psoriasis and Crohn's disease, ulcerative colitis and multiple sclerosis.