  With more and more exoplanets being detected, it is paid closer attention to
whether there are lives outside solar system. We try to obtain habitable zones
and the probability distribution of terrestrial planets in habitable zones
around host stars. Using Eggleton's code, we calculate the evolution of stars
with masses less than 4.00 \mo. We also use the fitting formulae of stellar
luminosity and radius, the boundary flux of habitable zones, the distribution
of semimajor axis and mass of planets and the initial mass function of stars.
We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 \mo,
and calculate the habitable zones of host stars, affected by stellar effective
temperature. We achieve the probability distribution of terrestrial planets in
habitable zones around host stars. We also calculate that the number of
terrestrial planets in habitable zones of host stars is 45.5 billion, and the
number of terrestrial planets in habitable zones around K type stars is the
most, in the Milky Way.
