Abstract: Based on the fractal viewpoint suggested by Mandelbrot,seismic activities (ML3.0) are studied for the Haicheng,Tangshan and Songpan earthquakes respectively before and after the occurrence of the large earthquake. The results show that a property of temporal-spatial distribution of seismic activities in a one or two-dimension space has the quality of statistical self-similarity at different length scales. Especially,it has a noninteger fractal dimension less than one or two. And this property is similar to that of one-dimension Cantor set or one-dimes ion continuum Cantor set. In particular,it is found that the fractal dimension is obviously different before and after the occurrence of the large earthquake. In general,that fractal dimension is lower before the quake than after.In this paper,the fractal dimension is considered to be a good physical quantity for describing the complexity in temporal-spatial distribution of seismic activities. Perhaps,it will be of important help in the prediction of large earthquakes.