In this paper, we describe an efficient coupled-domain nonlinear macromodeling method for microsystems, and present the study of electro-thermal effects on the pull-in characteristic for a fixed-fixed beam by using this approach. The macromodeling method applies both the Karhunen-Loeve/Galerkin technique and the Arnoldi technique. The compact models generated by these two techniques are successfully integrated and tested. The application of the macromodels are demonstrated and discussed. Experimental verification for the simulated results is also provided.