>>Metin,
>>
>>In SQL Server you can not create an index using a function. Use a date range when looking for a date, e.g.
>>

>>select * from myTable where DateField >=@Today and DateField < @Tomorrow
>>

>
>Thank you all,
>I'll use Naomi's way. I use in first variable today 00:00 and second variable today 24:59:59 . I'm afraid of about performance in other ways. I'm not sure if sql server will use index with other ways...

It will, if you use a field on the left side of the expression and whatever you want at the right side.
BTW there is no such time 24:59:59 :-)

I strongly recommend you to use Naomi's suggestion:

select * from myTable where DateField >=@Today and DateField < @Tomorrow --- Se here is only less that, not less than or equal

Because SQL Server keeps milliseconds also you may filter some records if you use this:

select * 
      from myTable
where DateField >=@Today and DateField <= @TodayAtMidNight

You may have this records:
01/01/2010 23:59:59.937 and that record will be filtered.