Online monitoring of the rotor angle stability of large interconnected power system is a complicated job due to peculiar dynamics. In this paper, a methodology for estimation of alternator rotor angle by using empirical modelled reactance, voltage and current phasors measured from phasor measurement units (PMUs) is presented. The empirical model for computation of synchronous reactance of slack and generator bus is proposed which is free from machine parameters. The effectiveness of proposed rotor angle approach is validated on IEEE 9-bus test system under various dynamic scenarios such as alternator bus and remote line to ground (LG) faults. The simulation results reveal that the proposed rotor angle estimation methodology has effectively computed the rotor angle of all interconnected alternator during various dynamic events. Thus, the proposed alternator rotor angle approach based on synchronized PMU measurements and empirical reactance can suite for online monitoring of rotor angle stability in complex and highly dynamic modern power system.