A method for forming flash memory devices is provided. The method includes providing a semiconductor substrate, which comprises a silicon material and has a periphery region and a cell region. The method further includes forming an isolation structure between the cell region and the periphery region. Additionally, the method includes forming an ONO layer overlying the cell region and the periphery region. Furthermore, the method includes removing the ONO layer overlying the periphery region to expose silicon material in the periphery region. The method also includes forming a gate dielectric layer overlying the periphery region, while protecting the ONO layer in the cell region. In addition, the method includes forming a polysilicon layer overlying the cell region and the periphery region.