A key structure of a keyboard includes a cap having a cover and a key post protruded under the cover, a key base which has a base board, restraining posts and a guiding post protruded on the base board with being arranged and spaced to surround a receiving space thereamong, and a resilient dome with a recessed passageway mounted between the cover and the base board to receive the key post and the posts in the passageway. The key post includes a base pillar and guiding ribs arranged around the base pillar. A guiding channel is opened in an inside of the guiding post to communicate with the receiving space and passes through a top of the guiding post. The key post is movably inserted in the receiving space with one guiding rib being slidably inserted in the guiding channel and the other guiding ribs being resisted against the respective restraining posts.