A numerical simulation is carried out to investigate the effect of inlet port cross-section geometry on the in-cylinder flow of a motored four-valve, single-cylinder, four-stroke internal combustion engine during the intake and compression strokes. The simulation is performed by solving the three-dimensional time-dependent compressible RANS equations in conjunction with the standard k - epsilon turbulence model using the commercial CFD software STAR-CD. To simulate the reciprocating piston motion, the governing equations are solved on a multiblock unstructured moving mesh. Two inlet port cross-section geometries are considered in the present study: circular cross section and elliptical cross section. The emphasis of the present study is on the tumble motion. The evolution of the flow structure and the quantified strengths of the tumble motions, namely, tumble ratio and cycle-averaged tumble ratio in the symmetry and offset planes for the two cases, are presented. It is found that inlet port with elliptical cross-sectional area is more conducive to tumble motion. DOI: 10.1061/(ASCE)EY.1943-7897.0000044. (C) 2011 American Society of Civil Engineers.