Surface structure of a stepped surface of Pt, Pt(31 1) (=2(100) -(111)), has been determined under potential control in 0.1 M HClO4 with the use of in situ surface X-ray scattering (SXS). The crystal truncation rods (CTRs) are reproduced well with the (1 x 2) missing-row model. Relaxation of surface layers, which is observed on the low-index planes of Pt, is not found on Pt(31 1) in the "adsorbed hydrogen region". CTRs at 0.10 (RHE) have the same feature as those at 0.50 V, showing that the surface layers of Pt(311) have no potential dependence. Scanning tunneling microscopy (STM) also supports the (1 x 2) structure of Pt(311) in 0.1 M HClO4.