A modular bed includes a frame assembly, a plurality of elongated support units, and at least one vibration assembly including a rack, a stretchable web, and a vibration motor. The rack is detachably mounted on two selected adjacent ones of the elongated support units. The stretchable web has a central portion suspended across a lower opening of the rack. After the through bore is stretched to permit insertion of a positioning plate of the vibration motor, resilient retraction of the through bore causes a neck portion of the vibration motor to be trapped in the through bore with a motor body and the positioning plate of the vibration motor to be in abutting engagement with upper and lower surfaces of the stretchable web, respectively.