Currently,　semantic　integration　became　an　attractive　area　in　several　disciplines,　such　as　information　integration,　databases　and　ontologies.　As　many　useful　data　are　stored　in　the　existing　database　management　system　of　enterprises,　the　ontology　learning　method　could　be　used　to　convert　relational　database　to　ontology.　Therefore,　one　of　the　main　challenges　in　the　research　of　data　integration　and　sharing　based　on　semantics　is　to　construct　the　mapping　between　relational　databases　and　anthologies.　Moreover,　the　use　of　manual　work　in　the　mapping　of　web　contents　to　ontologies　is　impractical　because　it　contains　billions　of　pages　and　the　most　of　these　contents　are　generated　from　relational　databases.　In　this　paper,　improved　mapping　rules　are　proposed　in　this　paper　to　convert　relational　database　schema　to　ontology.　The　corresponding　algorithm　is　put　forward　and　the　affection　of　algorithm　is　analyzed.　This　approach　is　effective　for　building　ontology　and　important　for　mining　semantic　information　from　huge　web　resources.