Provided is a cap structure for a vessel, which is coupled with the vessel to open/close the vessel. The cap includes an inner cap coupled with an upper end portion of the vessel, an outer cap fitted around an outer diameter of the inner cap, a circular part inserted into the mounting part, in which the circular part includes protrusions, which are smoothly curved, and first detachable protrusions formed at a position perpendicular to the protrusions, a deformation member to deform a shape of the circular part, and a component inserted into the inner cap and the circular part and having a first detachable groove circumferentially formed at a lower portion of an outer-diameter surface of a coupling part formed on the component. The cap opens/closes the vessel by fixedly attaching the component to the cap disassembled from the vessel or separating the component from the cap.