The resonance tunneling characteristic of sandwich structure made of two different single-negative materials is investigated by means of the transfer matrix method. Merging of the resonance tunneling frequency is found when the conjugate matched condition is satisfied. The resonance frequency has nothing to do with the layer thickness when the thickness ratio remains unchanged. The only influence is to cause the resonant peak to be much narrower and quality factor enhanced when layer thickness increases. The electric fields corresponding to frequencies of the resonance modes are found to be strongly localized at interface of two media. When the losses are considered, the losses make the symmetry of electric field distribution destroyed and transmittance depressed.