Disclosed herein is a method for forming an isolation film of a semiconductor memory device. According to the disclosure, in a pre-treatment cleaning process performed before a tunnel oxide film is formed, a SC-1 cleaning process is performed at a temperature ranging from 60° C. to 70° C. Therefore, oxide films in a cell region and a peripheral region are recessed even in the SC-1 cleaning process as well as a DHF cleaning process. A DHF cleaning time can be thus reduced. Accordingly, the method can minimize loss of a silicon substrate by DHF and can thus control the depth of a moat.