The authors have been developing facile preparation methods of precisely structure-controlled Pt nanocrystals (NCs) by liquid reduction procedures. We have also been adopting shape and size controlled Pt NCs as model catalysts to evaluate active sites of catalysts. This review deals with our following recent research topics. We have demonstrated that control of reaction circumstances (ex., solvent polarity, additive etc.) during formation of Pt(0) nuclei and their growth led to precisely structure controlled Pt NCs, such as Pt cube and nanowire. Recently, we have succeeded to prepare very small Pt cube and site-specific deposition of Ag at corner of Pt cube. Several Pt NCs with different shapes (cube, nanowire, tetrahedron, cubooctahedron) and sizes were applied as model catalysts for olefin hydrogenation to evaluate active sites. The new strategy based on advanced nanotechnology will be expected as a novel tool to design high performance catalysts with strictly controlled active sites.