Abstract: Based on the analysis of worms propagation in a close computer group, a new discrete worms propagation model is presented and compared with the real propagation data collected by CAIDA, it is found that the model reflects properly the active role of worms propagation. By extending further the model, the behavior of worms which propagate in the form of selective-random scan is considered and it is concluded that in a close computer group the greater the number of vulnerable computers varies in each and every small network, the quicker the worms propagate.