An investigations on mayflies are carried out in 1999 to 2000 through 17 sites in Heilongjiang province and 5 sites in Jilin province in Northeastern China (Manchuria). A total of 40 species (including the undetermined ten species) representing 21 genera in 10 families of Ephemeroptera are herein recognized. The result showed that 36 species occured from Heilongjiang and 20 species from Jilin, respectively. Nine among forty species, Ameletus costalis (Matsumura), Baetiella tuberculata (Kazlauskas), Ecdyonurus levis (Nav s), Ecdyonurus dracon Kluge, Heptagenia kyotoensis Gose, Heptagenia kihada Matsumura, Paraleptophlebia chocolata Imanishi, Serratella setigera (Bajkova), and Uracanthella rufa (Imanishi), are reported for the first time from Northeastern China. General habitat features and microhabitats of 22 collecting sites are described.