We study charge transport through molecular junctions in the presence of electron-electron interaction by using nonequilibrium Greens function(NEGF)techniques and renormalized perturbation theory.In the perturbation treatment,the zeroth-order Hamiltonian of the molecular junction is composed of a collection of single-impurity Andersons models,which act as the channels where charges come through or occupy,and the interactions between different channels are treated as the perturbation.With this scheme,the effects of molecule-lead interaction,electron-electron interaction and hopping interaction are included nonperturbatively,and the charge transport process can be studied in the intermediate parameter range from Coulomb blockade regime to coherent tunneling regime.The concept of quasi-particle is introduced to describe the kinetic process of charge transport,and then two electric current transport mechanisms,sequential tunneling and coherent tunneling,can be distinguished and calculated.As a test study,the Hubbard model is used as the molecular Hamiltonian to simulate dimeric and trimeric molecular junctions.Various nonlinear current-voltage characteristics,including Coulomb blockade,negative differential resistance,rectification and current hysteresis,are shown in the calculations,and the mechanisms are elucidated.