Let A be a positive bounded operator on a Hilbert space H, ·, · . The semi-inner product x, y A := Ax, y , x, y ∈ H induces a semi-norm · A on H. Let T A and w A (T ) denote the A-operator semi-norm and the A-numerical radius of an operator T in semi-Hilbertian space H, · A , respectively. In this paper, we prove the following characterization of w A (T ) 2010 Mathematics Subject Classification. Primary 47A05; Secondary 46C05, 47B65, 47A12.