  We propose a mixture evolution scenario to model the evolution of the radio
luminosity function (RLF) of steep spectrum AGNs (active galactic nuclei) based
on a Bayesian method. In this scenario, the shape of the RLF is determined by
both the density and luminosity evolution. Our models indicate that the density
evolution is positive until a redshift of $\thicksim 0.9$, at which point it
becomes negative, while the luminosity evolution is positive to a higher
redshift ($z \thicksim 5$ for model B and $z \thicksim 3.5$ for model C), where
it becomes negative. Our mixture evolution model works well, and the modeled
RLFs are in good agreement with previous determinations. The mixture evolution
scenario can naturally explain the luminosity-dependent evolution of the RLFs.
