The notion of fuzzy subalgebras with
respect to a $s$-norm is introduced, and some related results are
investigated. A kind of product, called {\it $S$-product}, and the
direct product of fuzzy subalgebras of $BCK$-algebras via a
$s$-norm $S$ are also defined, and some properties of the
$S$-product and the direct product of fuzzy subalgebras of
$BCK$-algebras with respect to  a $s$-norm are discussed.