AbstractThe crystal structure of poly(aryl ether biphenyl ether ketone ketone) (PEDEKK) was determined to comprise a two‐chain orthorhombic unit cell with dimensionsa= 0.778 nm,b= 0.606 nm andc= 2.375 nm by using wide‐angle X‐ray diffraction (WAXD). According to the orthorhombic system, the 12 reflections of this polymer were indexed. The crystallite size increases with increasing the crystallization temperature. The results of the degree of crystallinity (Wc,x) calculated from WAXD were compatible with those from density (Wc,d) and calorimetry (Wc,h) measure