Abstract: In this paper the statistical distribution of the time interval between two successive earthquakes is discussed, statistically tested, and shown that it obeys Weibull distribution f() =-1exp-(/).Actual calculation results show that in general the seismicoccurence fashionparametre, the p-value, is less than 1.0. This means that the pattern of seismic occurence is successive. Using some results of the reliability theory, the degree of seismic danger function D(t) =1-exp-(t/) is introduced, it is the probability of occurrence of an earthquake in timet. In this paper we let t = 1 day and call D(1) the degree of seismic danger, the D-value. The results indicate that the degree of seismic danger, the D-value showed a significant rising trend before all nine earthquakes of Ms5.8 in North China (1970-1984). The relation between , , D-value and the actual seismic activity, stability of the calculated D-value etc. are discussed. Besides, it is thought that the D-value is a function of stress. The other parametres of reliability theory have similar variations as the D-value.