  We report temperature and magnetic field dependence of the thermal transport
properties in single crystals of (Sr/Ca)MnBi$_2$ with linear energy dispersion.
In SrMnBi$_2$ thermopower is positive, indicating hole-type carriers and the
magnetic field enhances the thermopower significantly. The maximum change of
thermopower is about 1600% in 9 T field and at 10 K. A negative thermopower is
observed in CaMnBi$_2$ with dominant electron-type carriers and, in contrast,
the magnetic field suppresses the absolute value of thermopower.
First-principle band structure shows that the chemical potential is close to
the Dirac-cone-like points in linear bands. The magnetic field suppresses the
apparent Hall carrier density of CaMnBi$_2$ below 50 K. The large
magnetothermopower effect in (Sr/Ca)MnBi$_2$ is attributed to the magnetic
field shift of chemical potential
