In this paper, routing and wavelength assignment (RWA) algorithms whose optimum object is the minimum number of wavelength required (MNWR) in WDM All Optical Network were studied. Since the number of wavelength required (NWR) obtained by the existing two RWA algorithms is associated with the numbering order of the nodes in the network, a statistical method was proposed to modify the RWA algorithms. With statistical method, a smaller NWR can be acquired. And, it is the first time that the statistical method was proposed to compare the RWA algorithms. Two RWA algorithms can be compared by contrasting the distribution of the NWRs obtained by each RWA algorithm.