Abstract: To accurately predict the start-up pressures required in start-up process of gelled crude oil pipeline, it's very important to calculate start-up yield stress of gelled crude oil. On the basis of pipe flow loop experiment, pressures versus time at each location along loop during the controlled flow start-up are analyzed to redefine start-up yield stress of gelled crude oil in loop and propose a new way to calculate it. The start-up yield stress of gelled crude oil in loop can be defined as the shear stress that the gelled crude oil on the wall experienced in the end of experimental loop at start-up time under specific start-up flowrate. Based on the definition of the start-up yield stress, the pressure distribution expression in the start time can be obtained by applying polynomial fitting. In addition, through derivation of the expression at the extreme end of the loop, start-up yield stress of gelled crude oil with well-defined physical significances can be obtained.