Using the statistical entropy of the Dirac field of static spherically symmetric black hole, the Stefan-Boltzmann's law of static spherically symmetric black ho les is calculated, and we obtain a conclusion that the radiant emittance of a bl ack hole is proportionate to quartic power of temperature of the event horizon o f the black hole. It is found that the value of Stefan-Boltzmann constant in curved space-time is different from that in Euclidean space-time, and the constant has different value in different space-time.