DataTable in ASP.Net Session

I have to show users entered parameters in Asp.net Gridview example some values from dropdownlist textboxes and startDate EndDate etc . I am getting these values from user and add them to a Temporary dataTable . i am adding each row to DatTable on every add button call it goes fine for first time means for first row when i try to enter another row on Add button Click i overWrites on previous row and shows me only one row.