In this paper, a calculation method for calculating sensitivity factor for power system security based on dc power flow is done by simulation method using Matlab. The DC power flow simplifies the power flow calculations by making a number of approximations including: completely ignoring the reactive power balance equations, assuming all voltage magnitudes are identical per unit, ignoring line losses and ignoring tap dependence in the transformer reactance. Hence the DC power flow reduces the power flow problem to a set of linear equations.