Abstract: Considering the conditions that the nodes are unable to convert the wavelength and that the number of multiplex wavelengths is limited in optical fibres, a new routing/wavelength assignment (N-RWA) algorithm is proposed to solve dynamically the routing and wavelength assignment problem for the arrival of service request. In the new algorithm a rational fitness function is designed taking account simultaneously of the number of hops in a lightpath and the current network conditions and the genetic algorithm is in combination with least influence wavelength assignment algorithm, thus improving the conventional RWA algorithm. Simulation results showed that N-RWA can significantly reduces the blocking probability in comparison with the conventional RWA algorithm with balanced load kept on in the whole network.