Poly(ethylene terephthalate) (PET) and 2 thermotropic liquid crystalline polymers (LCPs) with different chain rigidity were blended to make in-situ composite fibers on a conventional melt spinning equipment. The addition of the LCP-I (60PHB-PET) with a less rigid chain has been found to lower the orientation of the as-spun fibers while the LCP-2 (VectraA900) with whole aromatic rigid chain has a reverse effect, as evidenced from the birefringence results. Both kinds of composite fibers with 5 wt % LCP have a good drawability. There is a diffraction peak characteristic of intermolecular packing of LCP on the wide-angle X-ray diffraction curve for the as-spun fibers containing LCP-2 but not the case for LCP-1. The morphology formed during elongational flow is highly dependent on the composition and rigidity of LCP. For the dispersed phases of LCP-1, it is relatively difficult to be elongated, whereas LCP-2 dispersed phases will be easily deformed into fibrils during melt spinning. The mechanical properties of the blend fibers containing the LCP-I component are inferior to those containing the LCP-2 component. For the fibers with discontinuous fibril morphology, a Halpin-Tsai equation could well be used to describe the elastic modulus of in-situ composite fiber with LCP-2.