The purpose of this study is to examine the effect of drillpipe rotation and cuttings size on hole cleaning efficiency in a horizontal annulus flowing with a non-Newtonian fluid. It adopts computational fluid dynamics (CFD) simulation to predict annular pressure losses, cuttings concentration profiles, and cuttings velocity profiles. A combination of Wen-Yu drag force model and k-epsilon turbulence model closely predicted experimental pressure loss data from literature. The results indicated that drillpipe rotation improves the transport efficiency of small cuttings of size 0.5 mm better than larger cuttings size of 5 mm. Annular pressure losses increased with increasing drillpipe rotation but decreased with increasing cuttings size. This study is very unique as it reveals various flow patterns that may occur during drilling operation with different cuttings size and drill pipe rotation speeds. It also serves as a guide to optimize drilling hydraulics for efficient hole cleaning in extended reach wells. Copyright © 2019 Inderscience Enterprises Ltd.