Direct torque control-based sensorless speed control (DTC-SSC) of a synchronous motor is a high performance and reliable approach for the control of speed and stator flux modulus. This paper proposes an analytical method to study the effect of error in estimation of stator resistance on the stability of a synchronous motor controlled via DTC-SSC approach, a reliable resistance estimator based on the angle between the stator flux and current vectors with no need to solve nonlinear equations, and a modified structure for sensorless speed control of synchronous motors based on this estimator. Simulation and experimental case studies validate the proposed stability analysis and the resistance estimator.