This work examines the PBT/PET sheath/core conjugated fiber, with reference to melt spinning, fiber properties and thermal bonding. Regarding the rheological behaviors in the conjugated spinning, PET and PBT show the smallest difference between their melt-viscosity at temperatures of 290degreesC and 260degreesC respectively, which has been thought to represent optimal spinning conditions. The effect of processing parameters on the crystallinity of core material-PET was observed and listed. In order of importance, these factors are the draw ratio, the heat-set temperature, and the drawing temperature. The crystallinity of sheath material-PBT, however, can be considered to be constant, independent of any processing parameters. The bulk orientation, rather than the crystallinity of PET core, dominates the tenacity of PBT/ PET sheath/core fiber. Moreover, heat-set treatment after drawing is recommended to yield a highly oriented conjugated fiber. With respect to thermal bonding, PBT/PET conjugated fibers processed via high draw ratio but low-temperature heat setting can form optimal thermal bonds at a constant bonding temperature of 10degreesC above the T-m of PBT.