Hi ,
Kindly I wonder whether have any idea about the difference between RedHat & Centos operating systems ? I totally know that Centos is a derivative of Redhat but I'm talking about the similarity in terms of administration ( are the commands same?) for instance If someone had installed Centos then do practicing on as a preparation for RHCE Cert. he can pass or not ( can he manage both of them in similar way ?) , thanks.. 
CentOS is built from Redhat's source code. The differences are almost non existent. The things you would miss would be possibly the /etc/redhat-release file as well as any redhat specific extensions, such as RH Cluster, which is an additional product anyhow.
CentOS would be a good practice environment for you. All of the RHCE elements described on the Red Hat web site would apply to CentOS as well as RHEL, although the curriculum and test have changed somewhat recently. One difference would be how a RHEL system subscribes to RHN or a Satellite for updates, but that was not part of the material covered by the exam I took and I don't think that's an element of the current RHCE program. The ultimate authority for what is on the exam is at Red Hat | Training | Exam objectives ? RHCE