Crystalline morphology in the blends of poly(ether ether ketone) (PEEK) and poly (ether imide) (PEI) was investigated by small-angle X-ray scattering. The lamellar thickness was found to be constant (approximate to 6 nm, independent of blend composition). The long period in 80/20 PEEK/PEI blend was greater than that in neat PEEK, whereas 50/50 PEEK/PEI blend showed intermediate value. This result suggests that a large amount of PEI impurity is excluded from interlamellar region in the high PEI content blend, whereas the exclusion is negligible and most PEI chains reside between lamellae in the low PEI content blend. The population of secondary lamellae, estimated by the interface distribution function analysis, was lowest in the 80/20 blend, suggesting that the formation of the secondary lamellae is restricted by the presence of the large amount of impurity between primary lamellae.