A method of fabricating a floating trap type nonvolatile memory device is provided. The method includes forming a cell gate insulating layer on a semiconductor substrate, the cell gate insulating layer being comprised of a lower insulating layer, a charge storage layer and an upper insulating layer sequentially stacked; thermally annealing a resultant substrate including the cell gate insulating layer in a temperature range of 810-100° C.; and forming a gate electrode on the thermally annealed cell gate insulating layer.