Abstract: The roles of magnetic islands were reviewed in two essential aspects of collisionless magnetic reconnection： are rapid dissipation from magnetic energy to plasma kinetic energy and generation of energetic electrons. The current sheet around the X line may be extended and unstable to the tearing mode instability. Secondary magnetic islands are generated and then consequentially interact each other in the extended current sheet. Such a nonstationary magnetic reconnection can maintain a large reconnection rate for a long time and efficiently dissipate magnetic energy. Electrons can be accelerated inside magnetic islands by Fermi and betatron mechanisms, as well as by reconnection electric field in the vicinity of the X line. The interaction of magnetic islands can result in further electron acceleration. These acceleration processes can make energetic electrons possess a power-law spectrum.