A heat dissipating device includes a heat sink (10), a heat pipe (20), a heat reservoir (30) thermally connecting with the heat sink through the heat pipe, and a fan (40) generating an airflow through the heat sink. The heat pipe includes an evaporating portion (202) attached to the heat sink and a condensing portion (204) attached to the heat reservoir. The heat reservoir is made of metal containing working medium, such as water, therein. The heat reservoir stores or releases heat based on the amount of heat generated by the CPU to realize a compensation to the increase or decrease of temperature of the CPU, whereby the change rate of the temperature of the CPU from idle to busy condition and vice versa can be more stable.