High-resolution transmission electron microscopy (HRTEM) studies were performed on Pt/C multilayers fabricated for x-ray mirror optics. The multilayers with d-spacing of about 4 nm were deposited either by dc-magnetron sputtering or by ion-beam sputtering on commercially available Si wafer substrates. Atomic resolution TEM observations and selected area electron diffraction (SAED) of cross-sections of multilayers were made by using several TEM apparatus including an ultra-high-voltage TEM with theoretical point resolution of 0.10 nm. The HRTEM and SAED studies showed that the multilayer consisted of amorphous C layers and polycrystalline Pt layers having a texture with the Pt <111> axes oriented normal to the interface. The Pt grain size perpendicular to the layers was of about the layer thickness, while the grain size along the layers varied in a range up to 10 nm. Detailed analysis of the HRTEM images indicated that the interface of the multilayer was basically defined as surface of the Pt crystal grains, and the interface roughness originated in an arrangement of the grains. Interface broadening observed in the image was primarily attributed to the averaging of the roughness due to Pt grains.