An efficient numerical integration algorithm is developed to evaluate the spatial-domain Green's function of vector and scalar potentials. A rigorous formulation for the Green's function in multi-layered media is derived in the spectral domain involving Sommerfeld-type integrals. This approach requires little analytic derivation for singular terms, and reduces the computation effort by using the weighted-average method to evaluate the oscillatory tail integrals. It is found that this algorithm is very efficient and suitable to model interconnects and microstrip elements for printed circuits/antennas with arbitrary size and any number of layers.