Based on a left-handed medium (LHM) structure unit with smooth dispersion, broadband property and of electrical small dimensions, a microstrip antenna (MSA) substrate is designed partially loaded by both the LHM and the conventional dielectric medium for the miniaturization of MSAs, in which the backward wave property of LHM is employed to compensate for the phase shift resulting from waves propagating in the conventional dielectric medium. Both the analytical and numerical results demonstrate that the physical dimensions of a 4.70 GHz MSA have been significantly reduced by 37.92%, and can be no longer proportional to the working wavelength, while its bandwidth and gain can be equivalent to that of a conventional MSA. All above results indicate the practical capability of LHM in the miniaturization of MSAs.