The new crystal structure of the intermetallic compound (Nd,Mg)2(Ni,Al)7 has been determined by scanning transmission electron microscopy and electron diffraction. This crystal structure is different from that (either 2H or 3R) usually reported for the intermetallic compound R2T7 (R: rare-earth elements and T: transition-metal elements) in the constitution of block layers. While the block layer for the crystal structure usually reported is of the R2T7 stoichiometric composition consisting of one R2T4 unit layer and two RT5 unit layers, the block layer for the new crystal structure consists of a sub-block layer of the RT3 stoichiometry (formed with one R2T4 unit layer and one RT5 unit layer) and a sub-block layer of the R5T19 stoichiometry (formed with one R2T4 unit layer and three RT5 unit layers), which alternately stack on top of each other. The crystal structure is described based on the order–disorder (OD) theory and the simplest crystal structures (polytypes) among many other polytypes belonging to the same OD family is deduced to be of the 2H-type with the space group of P 6 ¯ m 2 and of the 6R-type with the space group of R 3 ¯ m . The experimentally observed polytype corresponds to the second simplest form of the 6R-type.