Understanding thermalization and entropy production is one of the outstanding problem in theoretical physics. However, in integrable systems, we can explicitly solve the time evolution and observe thermalization to so called generalized Gibbs ensembles. In this talk, we demonstrate it in one-dimensional free Fermi gas, which are trapped in external potentials or a circle, through quantum quenches. We analytically compute local observables such as particle density and show that they always exhibit power law relaxation at late times. We find a simple rule which determines the power law exponent. Our findings are, in principle, observable in experiments in an one dimensional free Fermi gas or Tonk’s gas (Bose gas with infinite repulsion).