The asymmetry of the absorption curve of electromagnetically induced transparency by a resonant coupling field in -type three-level systems for Rb87 cold atoms is investigated. We find that it is the other excited state, separated by an interval of 814 MHz, that induces this phenomenon. The primary cause of it is the stimulated Raman scattering. We also find that the ratio between the heights of the absorption peaks on the two sides of the transparency window is proportional to the intensity of the coupling beam. The theoretical result agrees well with the experimental data.