A dielectric-loaded slot antenna fed by a finite-width substrate microstrip line is studied theoretically and experimentally by using the spatial dyadic Green's functions. Emphasis is placed on the analysis of the new feeding method, the finite-width substrate microstrip line, which is a versatile microstrip structure and can be made as a stand-alone device. The dielectric load is a hemisphere placed over the slot. The volume equivalence theorem is used to establish the theoretical model of the microstrip line. A simple excitation method for the microstrip line is introduced and modified to fit our problem. The spatial domain multilayered dielectric sphere dyadic Green's functions are used to obtain the scattered electromagnetic fields from the hemispherical dielectric load. The microstrip-line current, the microstrip-substrate electric field, and the equivalent magnetic current on the slot are obtained and used to calculate the impedance and the radiation pattern of the antenna. Computed impedances of the unloaded slot antenna and the loaded slot antenna show good agreements with either published data or measurement values. Analysis shows that the permittivities of the microstrip substrate have a significant effect on the impedance level of the antenna. It is found that electrically dense (high permittivities) substrates can give better impedance-matching between the feeding microstrip line and the slot antenna. The reduction of the antenna impedance by using a thinner substrate has also been demonstrated in our investigation.