var islemList = (from isl in entities.Islemler where (isl.KayitTarihi.Date >= dbas && isl.KayitTarihi.Value.Date <= dbit) selec
The .Date property is not supported in Linq to Entities (though it may be supported in other implementations of Linq).
I realize that you only want to compare the dates, but there is no real problem with comparing the datetimes if the dbas and dbit values are datetimes with time 00:00:00. You might have to offset the dates or use other inequality checks to get the proper interval but the comparison will work as you intend.
I would personally go with Jandek's solution.
Use EntityFunctions.TruncateTime.
if KayitTarihi
is a date column in DB (and dbas and dbit are DateTime), use:
var islemList = (from isl in entities.Islemler where (isl.KayitTarihi >= dbas && isl.KayitTarihi <= dbit) select isl);