The co-pyrolysis effect of municipal sludge (MS) and polyethylene terephthalate (PET) was estimated using kinetic analysis via thermogravimetric (TG) analysis and pyrolyzer-gas chromatography/mass spectrometry (Py-GC/MS). The co-feeding of MS and MS ash to PET pyrolysis decreased the PET decomposition temperature, from 427 °C to 409 °C by MS and to 420 °C by MS ash. The production amount of mono aromatic hydrocarbons (MAHs) on the pyrolysis of PET and MS was also increased by 2.5 times compared to the theoretical value by applying their co-pyrolysis. Additional use of MS ash for the co-pyrolysis of PET and MS further increased MAHs by 4.1 times compared to the theoretical value because of the radical enhancement and the catalytic effect of ash in MS. The amount of MAHs on the co-pyrolysis of PET and MS was increased by 9 times larger than the theoretical value for the co-pyrolysis of PET (0.5 mg) and MS (0.5 mg) at 500 °C by applying the increased reaction temperature (from 500 °C to 600 °C) and MS co-feeding amount (from 0.5 mg to 1.0 mg) to the pyrolysis of PET (0.5 mg).