  Let $\{S_i\}_{i=1}^{l}$ be an iterated function system(IFS) on $\mathbb{R}^d$
with attractor K. Let $\pi$ be the canonical projection. In this paper we
define a new concept called "projection pressure" $P_\pi(\phi)$ for $\phi\in
C(\mathbb{R}^d)$ under certain affine IFS, and show the variational principle
about the projection pressure. Furthermore we check that the unique zero root
of "projection pressure" still satisfies Bowen's equation when each $S_i$ is
the similar map with the same compression ratio. Using the root of Bowen's
equation, we can get the Hausdorff dimension of the attractor $K$.
