Typically pressure drop prediction of gas-liquid flow in pipes is carried out by mechanistic models. A proper prediction of the pressure drop requires prior identification of the flow pattern. Previous studies indicate (Gokcal, 2003 and 2008) that the most likely flow pattern for highly viscous oil is slug flow, which consists of a liquid slug region, followed by a bubble or film region. For highly viscous oils, a simplification of slug flow models is proposed based on experimental observations. This simplification is based on the fact that the pressure drop in the slug region is much larger than the pressure drop in the film region. The developed model shows fair agreement when it is compared against the data with oil viscosities from 40 to 580 cP. This model is applicable for two-phase flow Reynolds number up to 2000, which are common values in heavy oil production systems and does not require iterative solution.