A method of fabricating a flash memory device using a process for forming a self-aligned floating gate is provided. The method comprises forming mask patterns on a substrate, etching the substrate using the mask patterns as an etch mask to form a plurality of trenches, and filling the trenches with a first insulating layer, wherein sidewalls of the mask patterns remain exposed after filling the trenches with the first insulating layer. The method further comprises forming spacers on the exposed sidewalls of the mask patterns, filling upper insulating spaces with a second insulating layer thereby defining isolation layers, and removing the mask patterns and the spacers.