Abstract: The theoretical analyses of patent pools almost exclusively assume that only one patent pool is in one market， so this patent pool is a monopoly. In the other hand， the theoretical analyses also assume that before the formation of patent pools， there were many complementary patents in one market. Many substitutability patents， however， were not considered in one market. However these assumptions are far from the fact of patent pools. The probability model of the formation of the patent pools was developed at the cases that one market could have two patent pools and many substitution patents. Why the number of patents is a key factor in formation of patent pools was explained and it could be concluded that when building patent pools， the type of patents should not be considered， neither could the number of patents.

Key words: formation of patent pools, competitive patent pools, the number of patents, the type of patents, expected profit