A field observation on breeding pairs,breeding behavior and migratory time of Mute Swan(Cygnus olor)were conducted in Dalai Lake Nature Reserve from August to September in 2006 and April to August in 2007.The results showed that breeding birds arrived the Dalai Lake in early April and stayed there to later October,and the residence duration was 196 days(n=3).In the late of April,they began to court at any suitable place.Courtship occurred in the morning and each event of courtship lasted 172 s(n=8)in average.The birds mainly depended on displaying to protect and mark their territory.Six breeding pairs were observed in 2006 and 2007,all breeding pairs were successful in breeding and the nest size was 4,2,4,3,6 and 3 respectively in 2006,however,but in 2007,3 out of 6 pairs succeed in breeding,the hatchlings were 4,4,and 5,respectively.