An apparatus and a method for transmitting multimedia data in a broadcast system are provided. A method for constructing a payload for a multimedia frame to be transmitted via a heterogeneous network, in a broadcast system that supports an Internet protocol-based multimedia service, includes dividing the data so as to transmit data of at least one media type and constructing a data region based on each datum of the divided data, constructing a header region including information on the state of the data contained in the data region and/or information on the data region, and combining the data region and the header region to construct a payload for the multimedia frame.