A luminous pedal for a bicycle comprises a pedal, a luminous unit, and a rotary unit. The pedal has slots at two lateral sides respectively and a supporting portion in the middle. One end of the pedal is provided with an axial hole communicating with the supporting portion. The luminous unit has luminous portions and a coil seat. The luminous portions are disposed in the slots and the coil seat is detachably connected with a permanent magnet having a dentate part. The rotary unit includes an axial rod, a first gear, and a second gear. The first gear is disposed at the axial rod that is inserted into the axial hole. The second gear has a first disc and a second disc that are superimposed coaxially and the first disc is smaller than the second disc and the first gear. The two discs are engaged with the first gear and the permanent magnet. Thereby, the permanent magnet can be rotated rapidly by means of the differential ratio of the first and the second gears to produce enough electric power. Consequently, the luminous unit can be lightened stably.