Abstract: To study the relationship between the funnel viscosity and the apparent viscosity of oil-based drilling fluid, the flow patterns of oil-based drilling fluids in the marsh funnel viscometer were analyzed. Based on the structure of the marsh funnel viscometer, research was conducted to determine the possibility of apparent viscosity determination for oil-based drilling fluid by using the marsh funnel viscometer. In addition, the equations for funnel viscosity of fluids were highlighted and mathematic models for marsh funnel and apparent viscosity of oil-based drilling fluids were established. Lab and field tests were performed to verify the accuracy of the newly developed models. Research results showed that the maximum relative errors between the calculated apparent viscosity and the measured apparent viscosity of indoor and field tests were 6.8% and 8.2%, respectively. Both of these errors were in a reasonable range. During field tests, the oil-water ratio was increased from 70/30 to 91/9 with no observable impact on final results. Lab and field test results showed the apparent viscosity of oil-based drilling fluids could be determined accurately by using the newly developed prediction model. In fact, funnel viscosity could be used on the drilling site to facilitate the determination of the apparent viscosity of the oil-based drilling fluid on site.