Internet of Things (IoT) incorporates concepts from pervasive computing and enables iteration between people and objects. Location service is one of the primary services of IoT. However, existing techniques for indoor localization either have low accuracy or are too complex, requiring a high investment. This paper presents a technique of RFID indoor localization based on an analytical model of the Doppler Effect. From the Doppler frequency, due to the relative motion between the reader antenna and the tag, it is possible to estimate the spatial position of the reader antenna. Test results show the technique provides the location of the reader antenna with high accuracy and meets the requirements of low complexity and cost.