Crystal structures and morphology of a polymer containing alternated terephthalic acid (T) and isophthalic acid (I) Linked phenylene units in poly(ether ketone ketone) [PEKK(T/I)] have been investigated via transmission electron microscopy (TEM), electron diffraction (ED), wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC) experiments. When the polymer is crystallized at high temperatures (at and above 310 degrees C), only one orthorhombic crystal unit cell (form I) is found, and the crystal dimensions are a = 0.778 nm, b = 0.610 nm, and c = 3.113 nm, At the crystallization temperature below 200 degrees C, another orthorhombic crystal unit cell(form II) forms, having the dimensions of a = 0.417 nm, b = 1.108 nm, and c = 3.113 nm. Even at this low-temperature range, form II always coexists with form I except in the case of solvent-induced crystallization. A possibility of the existence of a new form (form III) is identified in a temperature range between 200 and 300 degrees C, and usually coexists with form I. This form also possesses an orthorhombic unit cell with dimensions of a = 0.641 nm, b = 0.610 nm, and c = 3.504 nn. Transformations from form II to both forms I and III require melting and recrystallization. Relatively regular-shaped lamellar crystals in PEKK(T/I) can be observed.