Size-controlled lead sulfide (PbS) quantum dots were synthesized by the typical hot injection method using oleic acid (OA) as the stabilizing agent. Subsequently, the ligand exchange reaction between OA and thioacetic acid (TAA) was employed to obtain TAA-capped PbS quantum dots (PbS-TAA QDs). The condensation reaction of the TAA ligands on the surfaces of the QDs enhanced the conductivity of the PbS-TAA QDs thin films by about 2-4 orders of magnitude, as compared with that of the PbS-OA QDs thin films. The electron transport mechanism of the PbS-TAA QDs thin films was investigated by current-voltage (I-V) measurements at different temperatures in the range of 293 K-473 K. We found that the charge transport was due to sequential tunneling of charge carriers via the QDs, resulting in the thermally activated hopping process of Arrhenius behavior.