A computer system includes a chassis, a motherboard, a heat sink and an airflow guiding duct. The chassis includes a chassis bottom wall, a first chassis sidewall, and a second chassis sidewall. The first chassis sidewall defines a first ventilation hole. The second chassis sidewall defines a second ventilation hole. The motherboard with a chip is secured to the chassis bottom wall. The heat sink is secured to the motherboard for cooling the chip. The airflow guiding duct is configured to guide airflow flowing from the second ventilation to the first ventilation hole. A fan is received inside the airflow guiding duct.