A method for adjusting setup default value of a basic input output system (BIOS) and a main board are provided. The main board offers a BIOS memory which includes a boot block and a main block. The boot block includes an adjusting table, and the main block includes an original setup default value and a dynamic table. The adjusting table is used to read the dynamic table, and the original setup default value is adjusted to be a customized setup default value according to the adjusting table and the dynamic table. Afterwards, the customized setup default value is stored in a setup value memory.