The equilibrium triple point and pressure and temperature phase diagram of polyethylene were obtained by in situ optical microscopic and X-ray observations of the melting temperature of hexagonal and orthorhombic isolated extended chain single crystals at high pressure. The melting temperatures of extended chain crystals still showed a ΔT dependence at high pressure, which has been neglected in previous studies. The triple point pressure and temperature were 0.5 × 108 - 1.5 × 108 Pa and 20-30°C higher than those previously obtained by the d.t.a. method. These results suggest that lamellar thickness dependence is important even in extended chain crystals to obtain equilibrium thermodynamic physical quantities of polymers. © 1992.