<p style='text-indent:20px;'>In this paper, we investigate the existence of pullback attractors for a nonclassical diffusion equation with Dirichlet boundary condition in <inline-formula><tex-math id="M1">\begin{document}$ H^2(\Omega)\cap H^1_0(\Omega) $\end{document}</tex-math></inline-formula>. First, we prove the existence and uniqueness of strong solutions for a nonclassical diffusion equation. Then we prove the existence of pullback attractors in <inline-formula><tex-math id="M2">\begin{document}$ H^2(\Omega)\cap H^1_0(\Omega) $\end{document}</tex-math></inline-formula> by applying asymptotic a priori estimate method.</p>