Sivakumar,
Max have made the correct suggestion, field-symbols will solve your problem.
DATA: gt_table1 TYPESTANDARDTABLEOF mara,
gt_table2 TYPESTANDARDTABLEOF mseg.
FIELD-SYMBOLS: <fs_table>TYPEanytable.
CASE'X'.
WHEN rad1.
ASSIGN gt_table1 TO<fs_table>.
WHEN rad2.
ASSIGN gt_table2 TO<fs_table>.
ENDCASE.
* now use <fs_table> as datatable to your function module.