The effect of temperature of the hydrothermal modification on the structure of Rare-earth Y zeolite (REY) was studied by positron annihilation spectroscopy. We measured the positron lifetime spectrum as a function of the temperature (300-800℃) of one hour hydrothermal modification for the REY zeolite after through pre-heated dehy-dration at 150℃. All lifetime spectra could be resolved into five components. The fifth lifetime component and its in-tensity were found to be related to the size and number of the secondary pores. The experimental results showed that the secondary pore in REY zeolite was produced by hydrothermal modification in some temperature range

and the largest size and the greatest quantity of the secondary pores were observed in the sample treated at 500℃ for 1 hour. The effect of hydrothermal modification on REY zeolite without pre-heated dehydration was also discussed.