The effects of blending polyethylene (PE), polystyrene (PS), poly(ethyleneterephthalate) (PET), a flexible polyurethane (FPU), and a car shredded fluff waste (CSF) on fluidity development of a bituminous coal during carbonization have been studied by means of high-torque, small-amplitude controlled-strain rheometry and in situ high-temperature H-1 NMR spectroscopy. The most detrimental effects were caused by PET and PS, which completely destroyed the fluidity of the coal. The CSF had a deleterious effect on coal fluidity similar to that of PET, although the deleterious effect on the viscoelastic properties of the coal were less pronounced than those of PET and PS. On the contrary, the addition of 10 wt % PE caused a slight reduction in the concentration of fluid hydrogen and an increase in the minimum complex viscosity, and the addition of 10 wt % FPU reduced the concentration of fluid hydrogen without changing the viscoelastic properties of the coal. Although these results suggest that these two plastics could potentially be used as additives in coking blends without compromising coke porosity, it was found that the semicoke strengths were reduced by adding 2 wt % FPU and 5 wt % PE. Therefore, it is unlikely that more than 2 wt % of a plastic waste could be added to a coal blend without deterioration in coke quality.