Effects of quenching agent on grain boundary precipitates and precipitate free zone （PFZ） in 7055 aluminum alloy have been investigated by transmission electron microscope （TEM）. Precipitation of equilibrium η phase was observed on grain boundary in air. In the room temperature and boiling water quenched alloys

grain boundary precipitates with chain continuous distribution are somewhat different in phase sizes

and average width of PFZ are about 33 nm and 43 nm

respectively

while grain boundary precipitates in air quenched alloy are obviously different in phase size

exhibiting discontinuous distribution

and the width of PFZ is about 102 nm. The formation mechanism was approached also.