Gamma ray sources observation with the ARGO-YBJ detector

Since November 2007 the air shower detector ARGO-YBJ is continuously monitoring the gamma ray sky in the declination band from −10° to +70° at energies E>0.5TeV.In this work we present the results of our observations of galactic and extragalactic sources during more than 3 years, focusing our attention on the Crab Nebula, the blazar Mrk 421 and the galactic extended source MGRO J1908+06, probably associated to the Fermi pulsar PSR J1907+0602.