Three-dimensional direct numerical simulations of the motion of a gas bubble rising in viscous liquids were carried out by a coupled level-set/volume-of-fluid (CLSVOF) method which is comprised of a best mix of the advantages of both the volume-of-fluid (VOF) and level-set (LS) methods. In our study, physical properties of real gas-liquid systems were introduced in the computations: the density ratio between both phases can be up to about 800 and the viscosity ratio reaches -50000. As a result, it will be presented that we can numerically predict three-dimensional bubble motion including nonlinear dynamics for the full range of bubble shapes and physical parameters (e.g. high Reynolds number) with good agreement with experiments. We shall describe the effectiveness and validity of our numerical (CLSVOF) method for two-phase flow.