We demonstrate tunable all-optical pulse compression and stretching via doublet Brillouin gain lines in an optical fiber. Tunable pulse compression or a stretching ratio of 1.47-0.43 accompanying a time delay is achieved by controlling the separation between two gain lines, for an input pulse train with a 40 ns width and a repetition rate of 5 MHz, in a 4 km silica fiber with a fixed pump power of 88.1 mW. The limitation of this pulse compressor or stretcher is also discussed.