Liquid-liquid flows in microchannels are important to microreactors/microfluidic devices that are used to carry out liquid-liquid reactions, extractions, emulsifications, etc. In this work, we report numerical investigations of drop/slug formation and flow regimes for liquid-liquid (oil-water) flow in microchannels. The Volume of Fluid (VOF) method was used to simulate the dynamics of water drop/slug formation in silicon oil, and the predicted drop/slug shapes/lengths were compared with previous literature measurements [Garstecki et al., Lab Chip 2006, 6, 437-4461]. The effects of flow rates of water and oil phases (0.019-0.417 and 0.004-0.14 mu L/s, respectively), channel size, liquid-liquid distributor (T-junction and Y-junction), and liquid viscosity on liquid-liquid flow regimes and slug lengths were investigated. The predicted drop/slug formation dynamics/slug lengths agreed satisfactorily with the aforementioned Garstecki et al. literature measurements for Q(water)/Q(oil) in the range of 0.1-1.7. However, for Q(water)/Q(oil) > 1.7, unlike the (long) slug flow reported in the aforementioned Garstecki et al. literature, a parallel flow was observed in the numerical simulations. The effect of wall adhesion (contact angle) on the flow regimes and slug lengths was also investigated. The experimentally validated computational model will be useful to simulate mixing, transport processes, and chemical reactions in microchannels.