This work provides an analytical derivation that a symmetric peak with a width narrower than that of the probing laser can appear due to the non-coherent superposition of two Fano-type peaks.It is well known that an ultranarrow line would be of great potential for accurate measurements in the physical sciences,which are the basis of fundamental physics and various high technology applications,such as the development of an atomic clock.Recently,the subnatural narrowing of an absorption line induced by various quantum interference phenomena has been investigated theoretically and experimentally,such as electromagnetically induced transparency effects in a tripod system[1] and coherent population oscillations in a A-type atomic system.