Etch pattern on surfaces of ZnO crystal has been observed by using an E.M. and an S.E.M. Two kinds of boundaries on which stacking orders of Zn and O atomic planes change have been developed on the plane {1010} by using etching technique. Hydrofluoric acid (~46%) was used as etchant. One boundary on which stacking of Zn and O planes changes from one order, -O.Zn-O.Zn-, to the other order, -Zn.O-Zn.O-, is a twin boundary. While, the other boundary on which stacking of Zn and O planes changes from the latter order to the former one is not considered as a twin one. Results of observation suggest to us that the decrease in the growth rate of the crystal needle is due to the generation of twin boundaries in the crystal