In this study, an in situ grafting/crosslinking method was employed to prepare form stable thermal regulating fibers (TRFs). The fibers were made from polyvinyl alcohol (PVA, as fiber polymer), polyethylene glycol (PEG, as phase change material) and butane tetracarboxylic acid (BTCA, as crosslinking agent) and spun by dry spinning. The addition amount of BTCA and thermal treatment temperature for preparation were optimized by studying the reaction kinetics of PVA/BTCA and PEG/BTCA composite systems. The structures and properties of PEG/PVA/BTCA composite fibers were characterized by differential scanning calorimeter (DSC), two-dimensional wide angle X-ray diffraction (WAXD) and scanning electron microscope (SEM). Results show that small amounts of BTCA can form adequate chemical bonds to fix PEG in the PEG/PVA/BTCA composite at proper temperatures in the thermal treatment. In composite fibers, the crystallization of PVA promotes the phase separation between PVA and PEG, and the crosslinkages confine the crystallization of PEG. In the as-spun fiber, the crystallization of PEG occurred along the normal direction of the fiber axis. During the stretching, the original orientation of PEG crystals in fibers was destroyed and then new crystalline orientation formed along the fiber axis. The cross-sections of the composite fibers are almost round; after being extracted, the fibers crosslinked by BTCA are denser and have fewer defects than un-crosslinked PEG/PVA fiber, which demonstrated that the PEG can be fixed steadily in PVA fiber matrix by the in situ grafting/crosslinking method. (C) 2015 Elsevier B.V. All rights reserved.