Offering a brief introduction to Upanishads and stating several mystical-philosophical issues, the present article seeks to investigate the semantic realm of Guru, types of Guru, the qualities of Guru and his disciples, the rituals and teachings of Guru, and their role in the way stations of mystic journey in the mystical thought of the Upanishads. No doubt, the Upanishads is one of the Hindu traditions in the philosophy and the mystical literature of the East, in which the Guru has a significant presence. So, it is not exaggeration to say that the Upanishads are collections of texts about the Guru's dialogues with his disciples. In Vedic philosophy, the Guru is depicted as each one of three gods of the Hinduism, i.e., Brahma, Shiva and Vishnu. In these texts, the Guru is more with an outer aspect, so he is often interpreted as Brahma and Rishi (sage), that is, the religious leaders who teach the eternal truths and secrets of sacrificial rituals to their new disciples. However, the Guru sometimes is of outer or inner aspect in the authentic philosophy of Upanishads. The Guru's outer aspect is not but the very Dhira and the undisturbed man, whereas the inner aspect of the Guru refers to the spiritual and the inner guide who manifests in the depth of new disciple's existence and guides him, after traversing the way stations of mystic journey.