Symmetric dimeric liquid crystals, alpha,omega -bis(4-alkoxy-phenylbenzyl-4-ylthio) perfluoroalkanes (nBFxBn), were newly synthesized and the mesomorphic phase transitions were investigated. The phase transition behaviours exhibited a dramatic dependence on the lengths of the two mesogenic groups and the fluoroalkyl spacer of the central part. These compounds showed layered mesophases like SmA, SmB and CrE, but it is noticable that twist grain boundary smectic A (TGBA) phase is found in non-chiral 2BF6B2 and 3BF6B3 compounds, which up to now is only found in chiral liquid crystal compounds.