  We study the evaporative cooling of magnetically trapped atomic hydrogen on
the basis of the kinetic theory of a Bose gas. The dynamics of trapped atoms is
described by the coupled differential equations, considering both the
evaporation and dipolar spin relaxation processes. The numerical time-evolution
calculations quantitatively agree with the recent experiment of Bose-Einstein
condensation with atomic hydrogen. It is demonstrated that the balance between
evaporative cooling and heating due to dipolar relaxation limits the number of
condensates to 9x10^8 and the corresponding condensate fraction to a small
value of 4% as observed experimentally.
