Try this:
For Each dr As DataRow In FirstDataTable.Rows
Dim results As System.Data.EnumerableRowCollection(Of System.Data.DataRow) = From myRow In SecondDataTable.AsEnumerable() Where (myRow.Field(Of String)("Id").Trim() = dr("Id").ToString().Trim())myRow
If results.ToList().Count <> 0 Then
MessageBox.Show("Row Exist")
End If
Next