Abstract: The invention discloses a method for controlling a rotation speed of a fan and a computer system for performing the method. The method includes the following steps. (a) In an operating system, an operating interface is provided for adjusting the current rotation speed of the fan. (b) When the current rotation speed is lower than a predetermined rotation speed, or a current temperature is higher than a predetermined temperature, a warning message is shown in the operating system. (c) After the current rotation speed is readjusted, all processing programs continue to be run. Furthermore, the invention also provides a hot key function for users to conveniently adjust the rotation speed of the fan under the operating system.