The integral equation (IE) method is commonly used to model time-harmonic electromagnetic (EM) phenomena. One of the major challenges in its application arises in the solution of the resulting illconditioned matrix equations. Here, we introduce a new domain decomposition (DD) based iterative method for the IE solution of time-harmonic electromagnetic problems. There are two major ingredients in the proposed IE-DDM: (a) the method is a type of nonoverlapping DD method and provides a computationally efficient and effective preconditioner for the dense matrix equation from the IE method; and, (b) The presented method is very suitable for dealing with multiscale EM problems. Each sub-domain has its own characteristics length and will be meshed independently from others. Numerical results demonstrate superior performance of the IE-DDM.