A syntactically correct DTD might be inconsistent in the sense that there exist no finite XML documents conforming to the structure imposed by the DTD. Inconsistent DTDs should be avoided. However, most consistency checking methods of DTDs without integrity constrains proposed now focus on whether there exists any valid XML documents conform to the DTDs and ignore consistency checking of local structures of DTDs, which result in the phenomenon that a consistent DTD may have sub-structures that no valid XML data could conform to. To solve this problem, notion of absolute consistency of DTDs is proposed and factors that lead to absolute consistency are analyzed. Algorithm for checking absolute consistency of DTDs is offered, which has linear time complexity.