I have installed Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 and Oracle 11g forms and reports....
My question is
Whether i can develop application using oracle forms 11g and using oracle 10g as backend connection......
If Possible then how to configure it......
That would depend on the exact version of Forms 11g you are referring to;
this document: http://www.oracle.com/technetwork/developer-tools/forms/oracle-forms-111210certmatrix-1886127.xls
is for forms 184.108.40.206 and indicates database 10.2.0.4 is certified with this version of forms. For other Forms versions this might be different (I don't know as I didn't look it up).
You whould do the SQL*Net config like you'd do it on any other oracle client:
Tnsnames.ora - Oracle FAQ
Sqlnet.ora - Oracle FAQ
except for reports where in the opmn.xml the TNS_ADMIN variable is set for the reports servers; so you'd have to place your config files under $INSTANCE_HOME/config as well.
TNS ADMIN - Oracle FAQ