Brushless doubly-fed induction generator (BDFIG) is a new type of ac induction machine and can be used for both grid-connected and stand-alone power generation. The conventional control methods for the BDFIG almost all rely on the encoder, which brings many limitations in terms of cost, complexity, reliability, and so on. This paper presents a new rotor speed observer (RSO) for the encoderless operation of the stand-alone BDFIG, which is based on a rotor position phase-locked loop (PLL) and a second-order generalized integrator (SOGI) and independent of any other machine parameters except for the pole pairs. Comprehensive experiments are also presented to validate the effectiveness of the proposed RSO.