Abstract: The conservation in sequence and structure of protein tyrosine phosphatases has been analyzed by aligning their sequences and superposed their three-dimensional topological structures.The results indicated that only three residues related closely to the function are conserved in sequence,but the core regions in their functional domains are strikingly similar in structure,and there are two conserved structural motifs,I.e.,βαβ and βαβα motifs.On the other hand,the topology of the active site in protein tyrosine phosphatases is also very similar.Thus,it is suggested that the very important residues for maintaining protein function are highly conserved,however,the residues for keeping protein structure are conservatively varied.In molecular evolution,three-dimensional structural conservation seems to be mainly expressed in holding some common secondary structural elements and common fold.