In a three-dimensional off-lattice model, Shakhnovich and Gutin's method of minimizing the Hamiltonian was applied to protein design. The results reveal that the number of the type of amino acids is one of important factors that determines the designability of a protein. For a chain made of 16 residues, the designed protein could fold into its native state from an arbitrary initial random coil conformation under appropriate conditions only when the number of type of amino acid increased to four.