Low resistivity n-type diamond films were prepared by boron and phosphorus co-doping with the ion implantation method. Hall effect measurements show that the samples have nearly equivalent carrier concentration while the Hall mobility and conductivity of B-P co-doped diamond films are higher than those of P doped diamond films. FTIR measurements show that the formation of B-H complexes passivates the acceptor of boron and prevents the phosphorus from passivating by boron. EPR and Raman measurements indicate that the B-P co-doped diamond films have more perfect lattice than those of P doped diamond films, which is benefit to improve the carrier mobility and decrease the resistivity.