For a class of (non-symmetric) diffusion processes on a length space, which in particular include the (reflecting) diffusion processes on a connected compact Riemannian manifold, the exact convergence rate is derived for $({\mathbb E} [{\mathbb W}_p^q(μ_T,μ)])^{\frac{1}{q}} (T \to \infty)$ uniformly in $(p,q)\in [1,\infty) \times (0,\infty)$, where $μ_T$ is the empirical measure of the diffusion process, $μ$ is the unique invariant probability measure, and ${\mathbb W}_p$ is the $p$-Wasserstein distance. Moreover, when the dimension parameter is less than $4$, we prove that ${\mathbb E} |T {\mathbb W}_2^2(μ_T,μ)-Ξ(T)|^q \to 0$ as $T\to\infty$ for any $q\ge 1$, where $Ξ(T)$ is explicitly given by eigenvalues and eigenfunctions for the symmetric part of the generator.