This paper proposes a new sensitivity factor to solve the real-time line flow calculation problem, which is called Jacobian Based Distribution Factor (JBDF). It is derived from the Jacobian matrix of the base case Newton-Raphson load flow solution, and kept constant during real-time line flow calculation. Different from the well-known distribution factors, such as Generation Shift Distribution Factor (GSDF), Generalized Generation Shift Distribution Factor (GGDF) and Z-Bus Distribution Factor (ZBD), it not only reflects the change of each bus injection active power, but also the reactive power. When the loading conditions change from the base case loads, either conforming or non-confirming changes in real and reactive power, it can fast and precisely compute the active line flows without the need of iterations. The proposed JBDF method is tested by IEEE 14-Bus System. The numerical simulation results demonstrate that the proposed method is very accurate and fast in computation. It is very suitable for real-time applications.