Low-temperature phase transitions in Rb2ZnBr4 single crystal were reinvestigated by means of neutron and X-ray diffractions. Below 116 K some superlattice reflections appeared at the same position (h/2, k/2, 0) as in other members of Rb2ZnCl4 group crystals, where h and k denote the Miller indices in the normal phase (space group Pmcn). The temperature dependence of the integrated intensity of the superlattice reflection changed below 78 K. However, the anomaly around 50 K reported previously was not observed. Below 116 K an incommensurate phase as reported for K2ZnCl4 was not detected. The space group of phase between 78 and 116 K was directly confirmed to be C1c1 by the verification of extinction rules of X-ray diffraction.