Abstract: Heavy oil has a special structural property due to the high molecular mixtures of colloid and asphaltene in it, and thus shows a feature of nonlinear flow. The start-up pressure gradient is a key indicator to study the nonlinear flow. Taking an oilfield as an example, some common measuring methods in China and abroad were compared and analyzed, and the optimal experimental method was selected to define the critical viscosity for the existence of start-up pressure gradient at different permeabilities. Typical curves were prepared. Finally, the relationship between the start-up pressure gradient and the mobility within the full mobility range of the oilfield was diagnosed. The experimental results show that the micro-flow displacement method is a practical method for measuring the start-up pressure gradient. The critical viscosity at which the start-up pressure gradient exists in the oilfield increases with the increase of permeability,but at a gradually-gentle rate. When the mobility is small, the start-up pressure gradient drops fast with the increase of mobility,and as the mobility increases steadily, the decline rate of start-up pressure gradient slackens.