Noise coupling from a working IC to the neighboring victim antenna is estimated by decomposition based reciprocity theorem. The overall problem is decomposed into two parts. In the first part, the IC is turned on and the victim antenna is turned off, which is defined as forward problem. The second part is called reverse problem in which case the victim antenna is excited while the IC is turned off. An imaginary Huygens's box is introduced above the victim antenna for evaluating the coupled power from noise source to the victim RF antenna. The noise source is characterized by Huygens's equivalent source in the forward problem, and the victim antenna is modeled in full-wave simulation tool. With tangential E and H fields obtained in both forward problem and reverse problem, the coupled power can be estimated by numerical calculation of reciprocity theorem. The method by using Huygens's equivalent source is first validated by simulation in the forward problem. Later a test board with an IC and patch antenna is further measured to validate the overall methodology.