Power DC-DC converters are notorious sources of electromagnetic interference in electronic products; therefore, suppression of EMI is an important topic in the design of switch-mode power converters. Conventional EMI suppression methods may employ LC filters and metal shielding, but this can significantly increase cost, size, and weight. In this paper we propose a chaos-based method to reduce peak EMI magnitude and spread the energy to a wide range of frequencies. This method is to add a chaos-modulation clock to a converter maintaining in regular period state, in order to create chaotic behaviors in the converter. Our results show that this method is effective in suppressing EMI in DC-DC converters without much impact on the performance of converters. System stability is also analyzed and discussed. Moreover, compared with conventional methods, this method can be easily implemented in CMOS technology.