Self-assembling of organic passivated nanocrystals has attracted a lot of interest recently. In this paper, the structural evolution of tetrahedral CoO nanocrystals is studied in situ using transmission electron microscopy. The as-prepared superlattices are Na(AOT)-passivated CoO nanocrystals, packed into monolayer and multilayer arrays on an amorphous carbon film. As the specimen temperature increased from 170 to 200 degrees C, the passivation layer is gradually evaporated/decomposed. For temperatures higher than 200 degrees C, the CoO nanocrystals experience a solid-state reaction, while the monolayer packing configuration is still preserved, but not the multilayers. The replacement reaction finishes at similar to 600 degrees C, and the final product is identified as mixed carbide nanocrystals of Co2C and Co3C.