An element a in a ring R has generalized Drazin inverse if and only if there exists b ∈ comm2(a) such that b = b2a, a − a2b ∈ Rqnil. We prove that a ∈ R has generalized Drazin inverse if and only if there exists p3 = p ∈ comm2(a) such that a + p ∈ U(R) and ap ∈ Rqnil. An element a in a ring R has pseudo Drazin inverse if and only if there exists b ∈ comm2(a) such that b = b2a,ak − ak+1b ∈ J(R) for some k ∈ N. We also characterize pseudo inverses by means of tripotents in a ring. Moreover, we prove that a ∈ R has pseudo Drazin inverse if and only if there exists b ∈ comm2 (a) and m,k ∈ N such that bm = bm+1a,ak − ak+1b ∈ J(R).