The vacancy formation energy in metal Al have been computed by a first-principles pseudopotential method. The dependence of the calculated vacancy formation energy on the supercell size and k point sampling set have been studied in detail. Our results show that the atomic sites in the supercell should reach about 100 in order to have a convergent theoretical value for the vacancy formation energy in Al. The theoretical value using a supercell of 108 atomic sites is in excellent agreement with experimental data.