A novel homologous series -3-[-4-n-alkoxy benzoyloxy] phenyl--4-methoxy benzoyl ethylene, which consists of 11 members, has been synthesised and the mesomorphic behavior studied in relation to the molecular structure. The pentyl to hexadecyl homologues are mesomorphic and the remaining homologues are non-mesomorphic. The pentyl, tetradecyl, and hexadecyl derivatives are only enantiotropic nematic, while the hexyl, octyl, decyl, and dodecyl homologue derivatives exhibit smectic mesomorphism in addition to the enantiotropic nematic phase. The transition and melting temperatures were determined by an optical polarizing microscope with a heating stage. Analytical and spectral data confirm the molecular structures. The texture of the nematic phase is of the threaded or Schlieren type and that of the smectic mesophase is a typical smectic A phase. The transition curves of the phase diagram behave in normal manner without showing an oddeven effect. The smectic and nematic thermal stabilities are 129.5 degrees C and 150.7 degrees C, respectively. The novel series is partly smectogenic and predominantly nematogenic with a middle-ordered melting type whose mesomorphic phase length varies from 7 degrees C to 46 degrees C. The mesomorphic properties are compared with other structurally similar homologous series.