We present an efficient approach to realize the spatial power combination for omnidirectional radiation via metamaterials in the two-dimensional case. We propose a radially anisotropic zero-index metamaterial which can always produce ominidirectional radiation, independent of the number and position of the sources inside the metamaterial. When the radial component of the permeability tensor is approaching zero and the wave impedance is equal to that of free space, waves emitted from all sources inside the metamaterial are transformed into perfectly cylindrical waves without any reflection, and powers from different sources can be combined together to enhance the omnidirectional radiation. We have designed and fabricated such a radially anisotropic metamaterial, and both numerical and experimental results demonstrate the spatial power combination with high efficiency. The proposed idea can be extended to the three-dimensional case to generate perfectly coherent isotropic radiation in nature, which does not exist now. Metamaterial is a unique approach to obtain such a high-efficiency spatial power combination for omnidirectional radiation and isotropic radiation.