The invention provides an electronic apparatus including a casing, a thermal sensor, a controlling unit, an air filter and a cooling fan. The air filter is detachably attached to the inlet of the casing. When the air filter is attached to the inlet of the casing, a partial structure of the air filter neighboring the thermal sensor blocks an airflow from flowing through the thermal sensor. In particular, when the air filter is attached to the inlet of the casing, or when the air filter is not attached to the inlet of the casing, the controlling unit always calculates a determined speed by applying the temperature sensed by the thermal sensor in a function of speed versus temperature, and controls the cooling fan operating under the determined speed to cool the electronic apparatus.