The rotational viscosity coefficient [Formula Presented] of [Formula Presented]-pentyl-[Formula Presented]-cyanobiphenyl in the nematic phase is investigated by combination of existing statistical-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMAs, [Formula Presented] is found to be a function of temperature, density, rotational diffusion coefficient, and a number of order parameters (OPs). The diffusion coefficient and the OPs were obtained from an analysis of a trajectory generated in a molecular dynamics simulation using realistic atom-atom interactions. In addition, a set of experimentally determined diffusion coefficients and OPs was used for evaluation of [Formula Presented]. Reasonable agreement between calculated and experimental values of [Formula Presented] is obtained. It is shown that near the clearing point [Formula Presented] is proportional to [Formula Presented], where [Formula Presented] is the second-rank OP. This limiting value of [Formula Presented] is in agreement with mean-field theory.