We prepared a homologous series of S-shaped oligomers, 4,4'-bis(omega-(2-(omega-(4-(cyanophenyl)phenyloxy)alkyloxy)phenyloxy)alkyloxy)biphenyl [I-(m,n)], and investigated effects of the spacer length on the phase transition behavior. I-(7,7), I-(8,8), I-(9,9) and I-(7,9) showed nematic (N) and smectic A (SmA) phases, whereas I-(6,6) and I-(6,9) showed only an N phase. The melting behavior was found to depend on the parity of the spacer (m). The liquid-crystalline phases of I-(8,8) and I-(6,9) are monotropic, however, those of I-(9,9) and I-(7,9) are enantiotropic. X-ray diffraction measurements of I-(7,7), I-(8,8) and I-(7,9) indicate that the S-shaped oligomers exist as a close-packed structure in the monolayer SmA phase.