Alchemy is the process of producing worthless substances into the best substances through chemical opus(work). On the surface, many of the alchemist's experiments can be depicted as work on transforming substances, but in reality, the alchemist's result is a product of the Unconscious. This study aims to explain the three phases of alchemy, Nigredo, Albedo, and Rubedo, through Michael Mayer's alchemical text, Atalanta Fugiens, and understand the transformation process by utilizing images that appeared from clients' sand play therapy. This study first described why alchemy, as the foundation for the psychology of the Unconscious, is important in sand play that deals with images. Next, Nigredo (blackening), the first phase of the alchemical process, is briefly described, and how the contents of Nigredo, such as chaos, dissolution, separation, division, corruption, death, and calcination, appear in sand play therapy. Next, the second phase, albedo (whitening), is described, and how the images of water and fire, which are representative images of albedo in the form of purification, sublimation, distillation, separation, descension, and coagulation, are revealed in sand play. Lastly, the phase of rubedo (reddening) in alchemy is described, and how the form of union (mandala or central image) in rubedo, which appears in the form of conjunction and rebirth, is revealed in sand play. The symbols revealed in alchemy are very valuable in amplifying the images that appeared in sand play therapy or dream analysis. In particular, the procedures found in alchemical opus are helpful in understanding the transformation process of personality.