Abstract: A ladder is composed of a plurality of rung units and each unit includes two hollow tubes and a rung connected between the two hollow tubes. The hollow tubes of the rung units have gradually reduced outer diameters so that the hollow tubes can be telescopically received to form a compact size. The rung is connected to the two hollow tubes of each rung unit by two connectors and each connector includes two buttons which can be pushed to release the interlock connection between the rung and the hollow tubes so as to easily collapse or extend the ladder.