We prove the existence of the O-U Dirichlet form and the damped O-U Dirichlet form on path space over a general non-compact Riemannian manifold which is complete and stochastically complete. We show a weighted log-Sobolev inequality for the O-U Dirichlet form and the (standard) log-Sobolev inequality for the damped O-U Dirichlet form. In particular, the Poincaré inequality (and the super Poincaré inequality) can be established for the O-U Dirichlet form on path space over a class of Riemannian manifolds with unbounded Ricci curvatures. Moreover, we construct a large class of quasi-regular local Dirichlet forms with unbounded random diffusion coefficients on path space over a general non-compact manifold.Suppose M is a n-dimensional non-compact complete connected Riemannian manifold, the Riemannian path space C o,T (M) over M is defined byC o,T (M) := {γ ∈ C([0, T ]; M) : γ(0) = o}, where T is a positive constant and o ∈ M. Let d M be the Riemannian distance on M, then C o,T (M) is a Polish space under the uniform distance d(γ, σ) := sup t∈[0,T ]d M (γ(t), σ(t)), γ, σ ∈ C o,T (M).