A method with pulsed electric or magnetic field to reduce the viscosity of crude oil is developed. Specifically, for paraffin-base crude oil, a magnetic field pulse can effectively reduce its viscosity for several hours, while, for asphalt- base crude oil or mixed- base crude oil, an electric field pulse can do the same. The method does not change the temperature of the crude oil; instead, it temporary aggregates paraffin particles or asphaltene particles inside the crude oil into large ones. This particle aggregation changes the rheological property of the crude oil and leads to the viscosity reduction. While this viscosity reduction is not permanent, it is suitable for many important applications, such as oil transport via deepwater pipelines, since it lasts for several hours and is repeatable.