The dechlorination processes and efficiencies of PCBs contained in transformer dielectric oils were investigated on three commercial-treatment companies. Alkaline dispersion method was used for dechlorination of PCBs mainly using molten metallic sodium or potasium hydroxide with PEG, and its dechlorination efficiency to whole congeners of PCBs was 98.94% on average. Homologues of highly-chlorinated biphenyls showed better efficiencies than those of less chlorinated ones, especially mono-chlorinated biphenyl in para-position (#2) was about 37 times enriched after chemical treatment. Co-planar PCBs' dechlorination efficiency was averaged as high as 99.78%, but 3,3',4,4'-TeCB was relatively low as 96.5%. It was suggested for better dechlorination efficiency to remove the water in transformer oil prior to chemical reaction and to stir the reagents more rapidly when using alkaline dispersion method.