We obtain upper bounds for the numerical radius of a product of Hilbert space operators which improve on the existing upper bounds. We generalize the numerical radius inequalities of (Formula presented.) operator matrices by using non-negative continuous functions on (Formula presented.). We also obtain some upper and lower bounds for the B-numerical radius of operator matrices, where B is the diagonal operator matrix whose each diagonal entry is a positive operator A. We show that these bounds generalize and improve on the existing bounds.