Phase behaviors of two homologous binary systems of 4-alkyl-4'cyanobiphenyl (nCB, n being the number of carbon atoms in the alkyl chain) and 4-alkoxy-4'-cyanobiphenyl (nOCB) were investigated. Their phase diagrams were drawn against an averaged chain length, n*. The phase boundary curves between neamtic (N) and smectic Ad (SmA(d)) phases in nCB/mCB (n = 3-6; m = 8, 9) binary systems were classified into four depending on four combinations of even and odd -numbered n and m; even/even, odd/even, even/odd, and odd/odd. The similar odd even effect was confirmed for nOCB/mOCB (n = 5, 6; m = 8, 9) binary systems. To elucidate the odd even effect on the phase diagrams, the SmA(d) structures were investigated through an analysis of n (n*) dependence of the layer spacing of the SmAd structure, ds(mAd), of neat nCB (n = 8-10), neat nOCB (n = 810), and nCB/8CB (n = 0-7) binary mixtures. The odd even effect, the behavior of OCB/8CB system, and the difference between nCB and nOCB were clarified based on their SmA(d) structures.