A method for manufacturing a flash memory includes providing a substrate with a sacrificial oxide layer, a sacrificial poly-Si layer, a hard mask layer and a trench exposing part of the substrate and filled with an oxide layer, later depositing a oxide layer conformally on the sacrificial oxide layer and the oxide layer, and afterwards removing the oxide layer on the sacrificial oxide layer and on the top of the oxide layer and the sacrificial oxide layer to form a spacer as a STI oxide spacer.