  We present results of numerical investigation on thermal flux in
Rayleigh-B\'{e}nard magnetoconvection in the presence of a uniform vertical
magnetic field. We have studied thermal flux in different viscous fluids with a
range of Prandtl number ($0.1 \le \mathrm{Pr} < 6.5$) and a range of
Chandrasekhar number ($50 \le \mathrm{Q} \le 2.5 \times 10^4$). The power
spectral density of the Nusselt number varies with frequency $f$ approximately
as $f^{-2}$. The probability distribution function of the fluctuating part of
the Nusselt number is nearly normal distribution with slight asymmetric tails.
For a fixed value the Rayleigh number $\mathrm{Ra}$, the time averaged Nusselt
number $\langle \mathrm{Nu} (\mathrm{Q})\rangle$ decreases logarithmically with
Chandrasekhar number for $\mathrm{Q} > \mathrm{Q}_c$, which depends on
$\mathrm{Ra}$ and $\mathrm{Pr}$. The reduced Nusselt number $\mathrm{Nu_r}$ $=$
$\langle \mathrm{Nu}(\mathrm{Q})\rangle/{\langle \mathrm{Nu}(0)\rangle}$ rises
sharply, reaches a maximum slightly above unity and then start decreasing very
slowly to unity as the value of a dimensionless parameter
$\sqrt{\mathrm{Ra/(Q~Pr)}}$ is raised. The probability distribution function of
the local thermal flux in the vertical direction is found to be asymmetric and
non-Gaussian with a cusp at its maximum.
