Simonkolleite (Zn5(OH)8Cl2·2H2O) nanodisks with a width of 40 nm have been successfully synthesized via a hydrothermal method using zinc chloride and ammonia as the starting materials. The conversion mechanisms from simonkolleite nanodisks to ZnO spindles under hydrothermal condition and to ZnO nanotablets under solid phase transformation were discussed respectively. The simonkolleite nanodisks obviously occur in hydrothermal system with the combination of a lower alkalinity, lower temperature (<363 K), a higher NH4+ ions concentration and in existence of Cl‐ anion. The morphologies, crystal phases and photoluminescence properties of as‐synthesized samples were investigated by field emission scanning electron microscopy, powder X‐ray diffractometer and fluorescence spectrophotometer. The photocatalytic activities of these as‐synthesized samples in the degradation of methyl orange have been demonstrated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)