A method and apparatus for controlling an amount of buffer data in a receiver of a data communication system, and a method and apparatus for playing back streaming data stored in a buffer, using an adaptive clock synchronization unit. The apparatus for controlling an amount of buffer data in a receiver of a data communication system includes a buffer for temporarily storing data, an adaptive rate determination unit for determining a rate at which data is used using operating conditions of the receiver and an amount of data stored in the buffer, and a data utilization unit for utilizing the data stored in the buffer at the determined rate. The method for adaptively controlling an amount of buffer data in a receiver of a data communication system, comprising determining a rate at which data stored in a buffer are used, considering operating conditions of a receiver and an amount of data stored in a buffer, and utilizing the data stored in the buffer at the determined rate.