We prove that: a regular ordered semigroup S is left simple if and only if every fuzzy left ideal of S is a constant function. We also show that an ordered semigroup S is left (resp. right) regular if and only if for every fuzzy left(resp. right) ideal f of S we have, f (a) = f (a2 ) for every a ∈ S . Further, we characterize some semilattices of ordered semigroups in terms of fuzzy left(resp. right) ideals. In this respect, we prove that an ordered semigroup S is a semilattice of left (resp. right) simple semigroups if and only if for every fuzzy left(resp. right) ideal f of S we have, f (a) = f (a2 ) and f (ab) = f (ba) for all a, b ∈ S .