A beta -hairpin peptide [Ace-Ile-Thr-Val-Asn-Gly-Lys-Thr-Tyr-Nme] in explicit water was studied with multicanonical molecular dynamics simulation. The obtained energy landscape was separated into two regions: deep energy wells and rugged surface. The deep wells consisted of beta -hairpin conformations with different hydrogen-bonding patterns, and the rugged surface had random-coils. Both the random-coil and the beta -hairpins were stable at 300 K. A mechanism was proposed for the beta -hairpin formation: non-oriented thermal fluctuation drives the peptide from the random-coil to a beta -hairpin. where the beta -turn is formed but the strand is partially disordered. This P-hairpin can switch to a different beta -hairpin, where the strand is ordered but the beta -turn is disordered.