We investigated the effects of plasma treatment on the sheet resistance of thin films spray-coated with graphene flakes on polyethylene terephthalate (PET) substrates. Thin films coated with graphene flakes show high sheet resistance due to defects within graphene edges, domains, and residual oxygen content. Cl2 plasma treatment led to decreased sheet resistance when treatment time was increased, but when thin films were treated for too long the sheet resistance increased again. Optimum treatment time was related to film thickness. The reduction of sheet resistance may be explained by the donation of holes due to forming pi-type covalent bonds of Cl with carbon atoms on graphene surfaces, or by C--Cl bonding at the sites of graphene defects. However, due to radiation damage caused by plasma treatment, sheet resistance increased with increased treatment time. We found that the sheet resistance of PET film coated with graphene flakes could be decreased by 50% under optimum conditions.