It is shown that in an open lambda type atomic system,the gain and dispersion of the probe field are quit sensitive to variation of spontaneously generally coherence （SGC）. The value of the parameter p reflects whether SGC effect exists and the size of SGC effect. Under certain condition, when p = 0. 002, electromagnetically induced transparency occurs; when p≠0. 002, lasing without inversion can be realized;when p = 0.95, high index of the refraction with zero absorption can be obtained.
