A device providing a secure execution environment is disclosed, wherein a microprocessor thereof is used for executing an unsecure application program and a secure application program. The unsecure application program is accessed from a system memory through a system bus and the secure application program is executed in a secure execution mode. The microprocessor includes an execution mode logic circuit that monitors the corresponding microprocessor and the state related to the potential interpolation, and converts the microprocessor from the secure execution mode to a degrading mode according to a first party in the state. The degrading mode is only provided for the BIOS command for execution. The secure non-volatile memory of the device is coupled to the microprocessor through a privatebus and stores the secure application program. The data transmission between the microprocessor and the secure non-volatile memory in the private bus is separated from the system bus and the corresponding system bus resources in the microprocessor.