The phase transition, crystallization kinetics, and morphology of monotropic liquid crystalline poly(ester-imide)s are investigated by synchrotron X-ray diffraction, differential scanning calorimetry (DSC), polarized light microscopy (PLM), and transmission electron microscopy (TEM). Synchrotron X-ray diffraction and DSC results show that the polyester-imide) with methyl side pendant group (10M) exhibits monotropic nematic and smectic A phases upon cooling from the isotropic melt, followed by a crystalline phase (crystal E structure). The results of isothermal crystallization kinetics show that the crystallization rate is faster in the smectic phase than in the nematic phase and the slowest in the isotropic phase. It is found that the crystallization in the nematic phase leads to lamellar decorated disclination structures upon TEM observation, indicating that the enhancement of nucleation dominates the acceleration of crystallization. Polyester-imide) without side pendant group (10H) forms only a crystalline phase upon cooling. However, the air-quenched sample shows a threadlike texture. TEM observation further reveals the trace of nematic disclination structures, confirming that the crystallization proceeds through nematic phase. In contrast, the sample, cooled with a slow cooling rate of 5 degreesC/min, exhibits the spherulite structure. The morphological variation of the polyester-imide) is discussed on the basis of competition between primary nucleation and crystal growth.