Example embodiments of the present invention relate to an etching solution, a method of forming a pattern using the same, a method of manufacturing a multiple gate oxide layer using the same and a method of manufacturing a flash memory device using the same. Other example embodiments of the present invention relate to an etching solution having an etching selectivity between a polysilicon layer and an oxide layer, a method of forming a pattern using an etching solution using the same, a method of manufacturing a multiple gate oxide layer using the same, and a method of manufacturing a flash memory device using the same. An etching solution including hydrogen peroxide (H2O2) and ammonium hydroxide (NH4OH) by a volume ratio of about 1:2 to about 1:10 mixed in water. In a method of forming a pattern and methods of manufacturing a multiple gate oxide layer and a flash memory device, a polysilicon layer may be formed on a substrate. An insulation layer pattern including an opening exposing the polysilicon layer may be formed on the polysilicon layer. The polysilicon layer exposed by the insulation layer pattern may be etched using the etching solution. A polysilicon layer pattern may be formed on the substrate using the etching solution.