In the paper a new approach to Adaptive Underfrequency Load Shedding, a procedure for protecting electric power systems from dynamic instability and frequency collapse is presented. It consists of two main stages. In the first stage, the frequency and the rate of frequency change are estimated by the nonrecursive Newton Type Algorithm. By using the simplest expression of the generator swing equation, in the second algorithm stage the magnitude of the disturbance is determined. The underfrequency load shedding plan Is adapted to the magnitude estimated, obtaining in this way a more efficient system operation during emergency conditions. Results of procedure testing are demonstrated through the dynamic simulations by using: a) a simple 3-machines test system and b) a 10-machines New England system.