Abstract: Based on the comparison and analysis on the sequences of cytochrome c family and some domain fragments in immunoglobulin family,the protein amino acid residue accessibilities have been calculated,and the relationship between amino acid residue accessibilities,protein sequences and three-dimensional structural conservativeness has been analyzed and discussed.The results indicated that all conserved residues in sequences have lower accessibilities,and the conserved residues with lower accessibilities are close correlative to keeping protein specific threedimensional structures.It is suggested that protein structural conservativeness is mainly expressed in their three-dimensional structures between evolutionary distant various protein molecules within families of proteins,and the conserved residues in sequences play an important role to keep specific three-dimensional structure in whole protein molecule.In addition,it is found that the conserved residues in sequences are generally located in the interior of whole protein molecular structure.This is why their accessibilities are lower.Of course,not all lower accessible residues are conserved,but the conserved residues are those with lower accessibilities.