A series of polycrystalline samples of Mg1-x(Zn0.5Al0.5)(x)B-2 (0less than or equal toxless than or equal to0.8) were prepared by solid state reaction method and their structure, superconducting transition temperature (T-c) and transport properties were investigated by means of X-ray diffraction (XRD), ac susceptibility, magnetization and resistivity. The double doping leads to decreases in both the lattice parameters a and c, and the T-c decreases with increasing dopant content. A systematical comparison with Al doped- and Li, Al double doped MgB2 of structure, superconducting transition and critical current density (J(c)) is made. It is suggested that the chemical pressure effect plays a more important role influencing the normal state transport and T-c than the change of carrier concentration, and the introduction of disorder into the Mg layers shows little influence improving the irreversibility field and J(c). (C) 2004 Elsevier B.V. All rights reserved.