Do anyone have any idea about mapping external Id in bulk API?

Hi Surbhi,
If you are doing upsert operation through Api then you need to map externalId.
In bulk Api you can map externalId by definng externalIdFieldName during job creation.

<?xml version=”1.0″ encoding=”UTF-8″?>
<jobInfo xmlns=”http://www.force.com/2009/06/asyncapi/dataload”>
<operation>upsert</operation>
<object>YourObject</object>
<externalIdFieldName>ExternalIdField</externalIdFieldName>
<contentType>CSV</contentType>
</jobInfo>