An elongated lamp tube has an elongated tube shell, two caps, a LED light source, a first power supply circuit, a second power supply circuit and a frequency switch circuit. The elongated lamp tube may be connected to first electrodes of a lamp fixture without a ballast or a lamp fixture with a ballast. The ballast converts an external power of an industrial frequency to a converted power of a working frequency. The frequency switch circuit is connected to the first power supply and the second power supply for blocking one of the first power supply circuit and the second power supply to supply driving current to the LED light source according to an input frequency of a received power.