{100} and {111} facets are typical for metal clusters having FCC structure. We have ever succeeded to obtain square Pt particles having {100} facet on their surface. However, there has not been an effective method to obtain Pt nanoparticles with the other facets, i.e., {111}. In this paper we report a novel preparation method of tetrahedral Pt nanoparticles, whose surface is controlled {111}, by using a thermosensitive capping polymer [poly(N-ethylacrylamide)].