An ultrasonic humidifier having stackable and detachable water reservoirs for easy expansion of its water storage capacity. A user can build the ultrasonic humidifier with a number of desired units of the stackable and detachable water reservoirs vertically to suit the usage and to accommodate the space in which the humidifier is used in. Each unit of the stackable and detachable water reservoirs has its own individual floater and draining device to control the replenishment of water to the unit it is attached to. The ultrasonic humidifier having stackable and detachable water reservoirs allows user to refill individual water reservoir unit separately.