The seismic gaps and their temporal-spatial parameters appearing around the source areas before 19 moderate-strong earthquakes in Gansu and its nearby region are studied using the R - T plots method. The result shows that 16 events among the 19 earthquakes formed seismic gaps before main shocks. The empirical formulas connecting the maximum spatial dimension and duration of the gaps with the magnitudes of the impending earthquakes are obtained from the temporal-spatial parameters of gaps of 16 earthquakes. It makes a basis to apply the ＂region -time -length＂ （RTL} algorithm to study anomalous seismic activities （ activation and quiescence ） before moderate and great earthquakes and its application in earthquake prediction in Gansu and its nearby region.