In a fan control method, consecutive temperature values are acquired at a time interval. Whether a current temperature value is higher than a threshold is determined. A difference between the current temperature value and a previous temperature value is calculated when the current temperature value is higher than the threshold. A fan is speeded up and the time interval is greatly shortened when the temperature difference is higher than a first trigger value. Only the time interval is slightly shorten when the temperature difference falls between the first trigger value and a second trigger value less than the first trigger value. Nothing to do when the temperature difference falls between the second trigger value and a third trigger value less than the third trigger value. The fan is slowed down and the time interval is extended when the temperature difference is lower than the third trigger value.