Disclosed are pyrimidine derivatives of the formula (I) and optical isomers thereof and a herbicide containing one or more of them as an active ingredient. ##STR1## where W is O or S; X is a lower alkyl group or a lower alkoxycarbonyl group; R is H, an optionally substituted lower alkyl group, an optionally substituted lower alkenyl group, an optionally substituted lower alkynyl group, an alkyali metal, an alkaline earth metal or an optionally substituted ammonium cation; R.sup.1 and R.sup.2 each are a halogen atom, an optionally substituted lower alkyl group or an optionally substituted lower alkoxy group; n is 3 to 5; m is 0 to 2; and is a single bond or a double bond.