Chern-Simons theory is a gauge theory with topological action . If we choose ISO(2,1) as a Lie group for this action , Chern-Simons action and 2+1-dimensional gravity are equivalent . With this choice we have equations of motion of general relativity and the gauge transformations will be the same as Lorantz transformation and diffeomorphism . Also 2+1-dimensional gravity with negative cosmolgical constant can be stated by Chern-Simons action with SO(2,2) and its gauge transformations . The inner product for SO(2,2) can be written in two ways so we have two Chern-Simons action for 2+1-dimensoinal gravity with negative cosmological constant , which sum of both actions will leads to normal; MARGIN: 0cm 0cm 0pt" By using Hamiltonian method for Chern-Simons action and obtaining the constrains and generators of gauge transformations we can find global charges for this theory and then with algebra of these charges we can find central charge . As will be seen algebra of global charges gives a central extension of its algebra . We study this algebra in two cases that leads to affine and Virasoro algebra . The algebra of the asymptotic symmetries’ generators of 2+1-dimensional gravity with negative cosmological constant is Virasoro . In Chern-Simons theory gauge transformations , with appropriate parameters , lead to Virasoro algebra . Thus asymptotic symmetries of 2+1-dimensional gravity with negative cosmological constant exist in Chern-Simons theory . As a result we can describe 2+1-dimensional gravity with negative cosmological constant by Chern-Simons theory by applying SO(2,2) as a Lie group .