This contribution examines in details the effects of dopants on the hole transporting properties of N,N′- diphenyl-N,N′-bis (1-naphthyl) (1,1′-biphenyl)- 4,4′diamine (NPB). Dopants for NPB are copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryle)- 4H-pyran (DCM1), 4-dicyanomethylene-2-methyl- 6-[2-(2,3,6,7-tetra-hydro-1H,5H- benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl0-5-(4- tertbutylphenyl)-1,3,4-oxadiazole (tBu-PBD) and 2,9-dimethyl-4, 7-diphenyl-1,10-phenanthroline (BCP). The effects of these dopants on the hole transport of NPB will be presented. Generally, the dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbitals relative to that of NPB. Traps are found to induce significant reduction in hole mobility. However, hole scatterers only alter the mobility slightly. Two different underlying charge transport mechanisms are proposed and then it is further examined by temperature dependent measurements.