Binary blends of poly(m-methylene 2,6-naphthalate) (PmN, m=2-8) were prepared by the solution-blending method and their miscibility was investigated with glass transition, crystallization, and melting behaviors, which were obtained from differential scanning calorimetry. It was determined that all the blends in which the difference in the number of methylene units between two component polymers was one were miscible. The blends in which the difference in the number of methylene units was two show divergent miscibility: two miscible, two partially miscible, and one immiscible systems. Among the blends in which the difference in the number of methylene units was three, two systems were partially miscible and the other two were immiscible. All the blends in which the difference in the number of methylene units was four or more were immiscible. It was thus found that the difference in the number of methylene units between two component polymers in the blend is the primary structural factor affecting the miscibility of a binary blend of PmN. The border between miscible blends and immiscible ones was found to be at differences in methylene-unit numbers of two or three.