A method and a system for diagnosing an apparatus are provided. The method for diagnosing the apparatus includes: determining whether the apparatus is to enter a diagnostic mode after a system is started; performing a diagnosis of the apparatus by executing a diagnostic proge apparatus is determined to enter the diagnostic mode; and performing a normal BIOS or EFI startup when the apparatus is determined not to enter the diagnostic mode. An accurate diagnosis of the apparatus can be realized by firstly determining whether the apparatus is to enter a diagnostic mode after a system is started, if yes, then the apparatus entering the diagnostic mode, and if not, then the apparatus performing a normal BIOS or EFI startup. Moreover, in case of a memory or PCI/PCIE bus error in the apparatus, since the diagnostic program is loaded prior to the BIOS startup, the fault can be diagnosed.