Microwave radiation from parametrically excited magnons was studied experimentally. Parallel pumping excitations were carried out on a ferromagnetic yttrium iron garnet (YIG). A high sensitivity microwave circuit was used to detect spectra of radiation microwave. At high pumping power, multi-frequency components are observed simultaneously. The spectra depend on static field, or magnetostatic modes. In order to characterize the timeseries of microwave radiation, a correlation dimension is estimated. These results show that a four-magnon scattering is important in a nonlinear relaxation process. (c) 2006 Elsevier B.V. All rights reserved.