The plasticity of coking coal is an important property influencing the strength of metallurgical coke. A molten coal viscometer with well defined flow field under a constant shear rate was designed, and the experimental procedures for reliable viscosity measurements were developed. The viscometer consists of a Searle-type concentric cylindrical cell, the rotor with the shortest possible length, a small split furnace of uniform internal temperature, and the hood for continuous vent of volatiles. The cell dimensions were determined to use the minimum possible amount of coal, while minimizing the end effect. A packing device for uniform packing of coal particles was also developed. The viscosities of 11 molten coals were determined. The trend in the minimum viscosity was in agreements with the trend in the maximum fluidity measured by Gieseler plastometer, but the viscometer showed narrower measurement range than the plastometer. The viscometer also gave the correct trends with the particle size and the packing density: the increase in coal particle size and packing density increased the plastic period and decreased the minimum viscosity.