  We study the real-time evolution of SU($2$) Yang-Mills theory in a $(3+1)$
dimensional small lattice system after interaction quench. We numerically solve
the Schr{\"o}dinger equation with the Kogut-Susskind Hamiltonian in the
physical Hilbert space obtained by solving Gauss law constraints. We observe
the thermalization of a Wilson loop to the canonical state; the relaxation time
is insensitive to the coupling strength, and estimated as $\tau_{\rm eq}\sim
2\pi/T$ with temperatures $T$ at steady states. We also compute the vacuum
persistence probability (the Loschmidt echo) to understand the relaxation from
the dynamics of the wave function.
