In a file structure for a streaming service and a method for providing a streaming service, a file structure includes a header object having basic information about a file and information for an application service, a data object synchronizing multimedia data with temporal information and storing it, and a key index object storing an offset and temporal information of a video block having a key frame in video blocks as a basis on a time axis for random access and reproduction. Accordingly, data can be transmitted quickly by not including unnecessary additional data, an application region can be expanded by including other media file besides a video and an audio files, because a random access and random reproduction by key index information are possible, it is possible to support various reproduction functions such as a fast play, a reverse play and random reproduction, etc.