In optical networks,the quality of service(QoS)is degraded by signal impairments incurred by non-ideal transmission. At the same time,different classes of service in the current optical network have different QoS requirement. So it is essential to develop intelligent routing and wavelength assignment(RWA)algorithms to combat the effects of impairments according to different QoS requirements. A new RWA algorithm is presented by combining the above two issues together. The simulation shows that the new algorithm can insure QoS of provisioned connections for clients with different QoS requirements.