Although studies on many owl species diets are common, there are only scarce data on the diet of the boreal owl from the lowlands of Eastern Europe. We have therefore studied its diet in one of the most important Polish population areas, the Knyszyflska Forest (north-eastern Poland). Pellets were gathered between February and June in 2006, 2012–2016 from tree hollows and the ground underneath. Altogether 178 pellets and six sets of pellet fragments were collected from 19 different sites. Eight mammal and eight bird species were identified among the 213 prey items found in the gathered material. Small mammals dominated, both in prey number (88.3%) and biomass (85.1 %). Bank vole proved to be the main prey (over 40% of prey numbers), while Microtus voles (27.3%), small passerine birds (11.7%) and shrews (7.5%) were important alternative prey. Diet range proved to be similar to other sites in Europe.