The present invention relates to a cap for hermetically sealing receiver driers, in which a cap body and a filter are separated and detachably engaged with each other and an O-ring is installed between the cap body and the filter, thereby facilitating the replacement of parts and the installation of the O-ring, favoring automatic assembly due to a tool insertion groove that can be formed long, achieving compactness of the cap due to the reduction of a total height of the cap when the cap body and the filter are engaged, improving a filtering effect by increasing the size of the filter with the cap being maintained in the same size, and facilitating engagement between the cap body and the filter and preventing the cap body from being released from a tank by discharging the air between the cap body and the filter.