A new algorithm has been developed for the layout based electro-thermal simulation of integrated circuits and MEMS, using simultaneous iteration. The general advantage of this method over simulator coupling is that, it is also suitable to cope with very fast changes. The usual drawback is however, that the thermal nodes have to be added to the nodes of the electrical network, rendering usually huge networks to be simulated. In our thermal modeling method the thermal system is represented by a matrix of Foster networks. The solution of these can be partly pre-calculated, partly separated from the solution of the electrical nodes, resulting in fast and accurate electro-thermal simulation. In the paper we present a case study, in which the electro-thermal simulation results of a realized MEMS electro-thermal converter show good match with the measured results.