The structure of a series of 1-alkyl-3-methylimidazolium fluorohydrogenate (RMImF(.)2.3 HF, R = alkyl group) room temperature molten salts has been studied by a high-energy synchrotron X-ray diffraction technique at high-energy X-ray diffraction beamline BL04B2 of Super Photon ring-8 (GeV) (SPring-8). The first sharp diffraction peak (FSDP) is observed at around Q = 1.8 Angstrom(-1) in the total structure factor of all the melts. The position of the FSDP is almost the same as that of a Bragg peak for solid EMImF(.)HF, indicating a layer like structure formed by ordering of the flat molecular cations, which give a correlation peak at 3.6 A in the total correlation functions. The results of ab initio molecular orbital calculations for the equilibrium geometries in comparison with the high-energy X-ray diffraction data suggest that H2F3- is a major component of the anions in EMImF(.)2.3 HF melt.