In this paper, a macromodeling procedure for coupled-domain MEMS devices with electrostatic and electrothermal effects is presented. Transient fully-meshed simulations using finite-element or finite-difference methods (FEM/FDM) for coupled-domain systems require tremendous computational cost. Therefore, in this work, we use the Karhunen-Loeve/Galerkin technique for extracting the macromodels that capture the system's nonlinear behaviors, such as the structural dynamics, the squeeze-film damping and the electrostatic actuation. In addition, using the Arnoldi-based technique, the thermal macromodels are reduced from the linear FEM/FDM models. The system dynamic behavior is successfully reproduced by using these macromodels. Compared with the fully-meshed model, the computational cost of the macromodels is reduced by at least a factor of 500 with less than 1% error. Experimental verifications for the simulated results are also provided.