Radiation modes for general planar multilayered structure are defined and their specific functional expressions are derived. Radiated fields from arbitrary field sources, surface and/or volume sources, can be uniquely expanded in terms of these radiation modes, the expansion coefficients of which are calculated using orthonormality properties for the corresponding radiation modes, once the source field distribution causing radiation is obtained by some numerical techniques such as, e.g., the FD-TD method. Practical application of this radiation mode theory is the evaluation of radiation, radiation into substrate region as well as into free space region, from multilayered Microwave Integrated Circuit (MIC) elements. General algorithm for evaluating radiated power and radiation patterns, using the present radiation modes together with the FD-TD method, is briefly summarized for multilayered MIC elements. Validity and accuracy of this method are discussed and numerically examined for ten-layered MIC microstrip line right-angled bends.