E-layer dominated ionosphere (ELDI) is referred to vertical electron density profiles having peak value at E-layer heights (Mayer and Jakowski, 2009).In this paper, characteristics of ELDI were investigated with the help of field-aligned measurements of EISCAT/ESR radars during 2009-2011.ELDI events were identified with simple but reasonable criterions, in which a minimal event duration was required to exclude possible "fake" events induced by random errors in measurements.It was found that ELDI were observed more often in winter than other seasons.In winter, occurrence of ELDI peaks around mid-night at auroral latitude, while it reaches its maximum around geomagnetic local noon at ESR.Our results indicate that ELDI seems not to be so regular in the nighttime auroral zone, being contrary to what claimed by previous results inferred from radio occultation measurements.Discrepancy in duration of ELDI events observed by the two radars is remarkable, being 23 minutes at Troms(O) on average and about a half of it at Svalbard.During the presence of ELDI, average thickness of E-layer showed undetectable variations at the two sites, as well as HmE and the ratio of NmE/NmF.We attribute the main reason of ELDI formation to the combination of extra E-layer ionization caused by particle precipitation and density depletion in F-layer caused by plasma convection in polar region.