Abstract: Electrospray ionization mass spectrometry and circular dichroism were used to investigate the factors which affect the motif of the c-myc promoter quadruplex, such as annealing. ESI-MS and CD spectra indicated that annealing could induce the conformational transition of the quadruplex from a monomer to a dimer, and both were parallel structures. Ion mobility mass spectrometry confirmed the existence of dimers, and also indicated that quadruplex dimers are preferred conformations for the c-myc sequences after annealing.