The second harmonic generation (SHG) of a femtosecond optical frequency comb (FOFC) has been studied. This work focuses on the SHG frequencies that are generated by the mixing of even-numbered frequency components from the original comb with odd-numbered components. It is observed that the generation of those frequencies is the reason the original FOFC and FOFC-based SHG signal have the same repetition frequency. The theoretical derivation agrees with the result of an optical experiment. Our results may be of use with the high-harmonic-generation process and FOFC-based SHG applications, including high-resolution spectroscopy, attosecond pulse generation, and precision length measurement.