Taking use of the interplays of rare-earth metal with C=C double bond, we successfully achieved allylic C-H alkylation of internal or terminal alkenes (pKa value from ca. 30 to 45) with styrene derivatives as well as dimerization of terminal and internal alkenes by cationic imidazolin-2-iminato scandium alkyl complexes. Moreover, this activation strategy was extended to benzylic C-H activation of toluene derivatives. Experimental studies and theoretical calculations support the in-situ formation of allylic rare-earth metal species. A possible catalytic cycle involving the assistance of Lewis base was proposed to understand the reaction mechanism and E/Z ratio of crosscoupling of internal alkenes with styrene derivatives. This work provides a new platform for the allylic C-H functionalization of internal alkenes. The development of chiral imidazolin-2-iminato rare-earth alkyl complexes and their further utility in other related C-H functionalization reactions are in progress.