Abstract: A conventional rotary viscometer was restructured available to variable speeds to measure the rheological characteristics of melts, e.g., the oil viscosities with different standard grades. The reproducibility error of this viscometer and the total error of measurements, i.e., the maximal permissible measuring error of Newtonian fluid in measurement process are given. The maximal permissible error caused by neglecting the effect of yield stress on viscous factor was determined through the analysis of measured data. An approach to determine whether a melt is non-Newtonian fluid and whether there is a yield stress in the non-Newtonian fluid was further given. The results provide a necessary basis for further research on the rheologic characteristics of melts.