Structural changes accompanying a cubic-orthorhombic transition in K2Mn2(SO4)(3) were investigated by high-resolution neutron powder diffraction. The structure parameters of cubic (P2(1)3) and orthorhombic (P2(1)2(1)2(1)) phases were precisely refined by the Rietveld method with intensity data measured at 300 K and 40 It, respectively. In the cubic phase, sulfate ions exhibit dynamic orientational disordering. Six oxygen atoms surrounding each Mn2+ ion form two kinds of highly distorted octahedra, whose degrees of distortion are remarkably different from each ol;her. On the other hand, sulfate ions are fully ordered in the orthorhombic phase. We conclude that the distortion of the MnO6 octahedra and short Mn-O distances in the cubic phase are closely related to the mechanism of the phase transition. Results of neutron diffraction experiments at 31 different temperatures support the idea that the orthorhombic --> cubic transition proceeds via the nucleation and growth of the cubic phase within an orthorhombic matrix.