Three-dimensional particle size segregation structures of binary mixtures of six different size ratios (SR = 1.423.37) in a rotating drum are studied. The formation of two smaller particle satellites around the central smaller-particle rich band after the band formation core-thickening mechanism is reported for the first time. The binary mixtures of six SRs show three satellite shapes, including the small bump shape, the axe shape, and the hemisphere shape. Except for the binary mixture of SR = 2.01 with the axe satellite shape, the satellite size increases with the increasing of the SR value at the same bed depth. The degrees of mixing of binary mixtures of six different SRs at different bed depths are analyzed using the Lacey mixing index. The degree of mixing at the bed surface and close to the drum cylindrical wall can be explained by the drift-diffusional model of Savage (1993). (C) 2011 American Institute of Chemical Engineers AIChE J, 2012