Abstract: A method for adjusting a virtual resource used in a virtual system is provided. The method comprises the following steps. At least one virtual machine is provided in the virtual system, and the at least one virtual machine has maximum and minimum thresholds of I/O (Input/Output) loading and maximum and minimum thresholds of operation loading. An I/O loading amount and an operation loading amount of the total loading amount of the virtual system are retrieved. The relationship of the I/O loading amount and the maximum and minimum thresholds of I/O loading is determined to adjust the number of the virtual machine, and the relationship of the operation loading amount and the maximum and minimum thresholds of operation loading is determined as well to adjust an unit resource amount of the virtual machine.