The temperature dependence of the elastic modulus E1 of the crystalline regions of poly(ethylene 2,6-naphthalate) (PEN-2,6) in the direction parallel to the chain axis has been investigated up to 228-degrees-C by X-ray diffraction. The E1 value of PEN-2,6 was 145 GPa at room temperature, which is about 40% greater than that of poly(ethylene terephthalate) (E1 = 108 GPa). The axial chain contraction coefficient was constant from 22 to 228-degrees-C, and correspondingly the value of E1 remained unchanged up to 228-degrees-C. The deformation mechanism is considered to remain unchanged up to 228-degrees-C and the skeletal conformation of PEN-2,6 in the crystalline regions is stable against heat. It is reasonable to conclude that stress is homogeneous throughout the sample.