A boot up method makes an electronic system boot up by a processor according to a boot code in a NAND flash memory and includes the following steps. First, the flash memory storing a boot code or boot codes is provided. Next, a first boot code is copied to an XIP memory in response to a hardware reset signal. Then, the processor executes the first boot code in the XIP memory and thus makes the system boot up. Next, whether the system boots up successfully is judged after a time delay. When the system fails to boot up, the system is reset and a second boot code is copied to the XIP memory. Thereafter, the processor executes the second boot code in the XIP memory and thus boots up the system. If the system still fails to boot up, the above-mentioned steps are repeated until the system boots up successfully.