  We construct a simple model for a magnetic reconnection heated corona above a
thin accretion disk in AGNs and Galactic black hole candidates(GBHCs). The
basic assumptions are that (1) the magnetic reconnection heat is cooled down
overwhelmingly by Compton scattering in the corona and that (2) thermal
conduction is dominantly cooled by evaporation of the chromospheric plasma in
the disk-corona interface before Compton cooling sets in. With these two basic
equations as well as equi-partition of magnetic energy with gas energy in the
disk, we can consistently determine the fraction of accretion energy dissipated
in the corona without free parameters, and thus determine the temperature and
all other quantities in both the corona and disk for given black hole mass and
accretion rate. Then, we calculate the luminosity contributed from the disk and
corona and the coronal flux weighted Compton $y$ parameter. It is found that,
at a low luminosity (less than 0.1 Eddington luminosity) the spectrum is hard
with energy spectral index of around 1, while at a high luminosity (larger than
0.1 Eddington luminosity) the spectrum can be either soft or hard. We also find
that the situation is almost same for supermassive and stellar-mass black
holes. These features are consistent with observations of AGNs and GBHCs.
