Flow-induced preordering or precursor(FIP) has been studied in a series of lightly cross-linked high-density polyethylene with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering(SAXS) and wide-angle X-ray diffraction(WAXD) measurements. Based on the incipient strains of SAXS and WAXD signals during extension in a large temperature range, strain-temperature diagrams for flow-induced preordering and nucleation were constructed and revealed that flow-induced crystallization(FIC) undergoes two stages: melt-precursor transition(MPT) and precursor-nuclei transition(PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain; these embryos have shape and structure that are related to those of the corresponding critical nuclei. With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk free energy(?H) and a much lower surface free energy(?e). In extreme cases(high temperature), the ?e of FIP can be negligible. The quantitative estimation of the thermodynamic parameters suggests the existence of variant FIPs, which plays a vital role for the subsequent progress of PNT and the whole process of FIC. Flow-induced preordering or precursor (FIP) has been studied in a series of lightly cross-linked high-density polyethylene with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle X Based on the incipient of SAXS and WAXD signals during extension in a large temperature range, strain-temperature diagrams for flow-induced preordering and nucleation were constructed and revealed that flow-induced crystallization (FIC) undergoes two stages: melt-precursor transition (MPT) and precursor-nuclei transition (PNT). At different temperatures, FIP with different inner structures and morphologies can be induced by strain; these embryos have shape and structure that are related to those of the corresponding With the strain-temperature diagrams, the thermodynamic properties of FIP are deduced, which shows that compared with the relative nuclei the FIP always has a lower bulk The quantitative estimation of the thermodynamic parameters suggests the existence of variant FIPs, which? e of FIP can be negligible. plays a vital role for the subsequent progress of PNT and the whole process of FIC.