A high temperature viscometer was designed and constructed for the investigation of coal slag viscosity under gasification conditions. Using a Searle-type concentric cylinder viscometer configuration, a unique design with a square shape on the outside and cylindrical on the inside was used for the cup. The dimensions of the cell (the cup and rotor) were determined based on numerical simulations of the rotating flow so as to minimize the end effects. The cell, made of high-density alumina, was placed in a high temperature box furnace using a support system, and two gas-feed lines were installed From the bottom of the furnace to control the gas atmosphere. The slag samples were fed into the furnace using a hole made in the top and the rotor was connected to the viscometer head. Two coal slags, one glassy and another crystalline, were used to test the viscometer. The reproducibility of the glassy slag was excellent, as demonstrated by the overlapping results of repeated measurements. When the reproducibility of the crystalline slag was compared using T(cv), the difference between the two measurements was found to be 17 ℃, which is an acceptable result for this type of slag.