Disclosed is an optically active compound represented by the formula: ##STR1## wherein R.sub.1 and R.sub.2 are alkyl, alkenyl or alkoxy-alkyl, which may be substituted by halogen, X and U are a direct bond, --O--, --COO-- or --OCO--, Y and Z are a direct bond, --COO--, --OCO--, --CH.sub.2 O-- or --OCH.sub.2 --, A.sub.1 and A.sub.2 are phenylene, cyclohexylene or a phenylene- or cyclohexylene-containing group, p and q are 0 or 1, C* is an asymmetric carbon atom, with the proviso that when Z is a direct bond, q is 0, R.sub.2 is alkyl, Y is a direct bond, --OCO-- or --OCH.sub.2 --. The optically active compound is used as a ferroelectric liquid crystal or an additive to a ferroelectric or non-ferroelectric liquid crystal.