  Gamma Ray Bursts (GRBs) are found at much higher redshifts (z>6) than
Supernova Ia (z~1), and hence, they can be used to probe very primitive
universe. However, radiation mechanism of GRB remains a puzzle, unlike
Supernova Ia. Through comprehensive description, both empirical and physical,
we shall discuss the most likely way to use the constituent pulses of a GRB to
find the radiation mechanism as well as using the pulses as luminosity
indicators.
