The microstructures of a dealuminated zeolite Y were observed with a transmission electron microscope (TEM) under a very low electron dose condition using a high sensitivity SSC camera for the purpose of elucidating the formation mechanism of the mesopores, Mesopores were distributed three-dimensionally, They were closely related with the preexisting (111) twinning planes. The possibility of controlling the distribution of mesopores by controlling the distribution of twins during synthesis is also mentioned.