  Let $(A, \frak m)$ be a noetherian local ring with maximal ideal $\frak{m}$
and infinite residue field $k = A/\frak{m}.$ Let $J$ be an $\frak m$-primary
ideal, $I_1,...,I_s$ ideals of $A$, and $M$ a finitely generated $A$-module. In
this paper, we interpret mixed multiplicities of $(I_1,..., I_s,J)$ with
respect to $M$ as multiplicities of joint reductions of them. This generalizes
the Rees's theorem on mixed multiplicity (Theorem 2.4). As an application we
show that mixed multiplicities are also multiplicities of Rees's superficial
sequences.
