A heat dissipation system used in an electronic device includes a casing, an inlet, an outlet, two thermal sensors, at least one fan, and a microprocessor. The fan is disposed in the casing and configured to guide airflow into the casing through the inlet and out of the casing through the outlet. The two thermal sensors are disposed in the casing and respectively located near the inlet and the outlet to measure a first temperature and a second temperature. The microprocessor has at least one relation table, which is a function of ratio of the second temperature to the first temperature, between the first temperature and speed of the fan. The microprocessor selects one relation table in accordance with the ratio of the second temperature value to the first temperature value to control the speed of the first fan according to the first temperature.