Apparatus and method for guaranteeing a Quality of Service (QoS) traffic rate in a mobile communication system are provided. The scheduling method for guaranteeing a QOS traffic rate in a mobile communication system includes calculating a total sum of traffic rates based on a channel change and a total sum of sustained traffic rates; calculating a weighted traffic rate using the traffic rate based on the channel change and the sustained traffic rate when the sustained traffic rate does not meet the QoS traffic rate; and determining a service priority using the weighted traffic rate. Therefore, the minimum reserved traffic rate and the maximum sustained traffic rate can be assured.