  We study the single transverse spin asymmetry of prompt photon production in
high energy proton-proton scattering. We include the contributions from both
the direct and fragmentation photons. While the asymmetry for direct photon
production receives only the Sivers type of contribution, the asymmetry for
fragmentation photons receives both the Sivers and Collins types of
contributions. We make a model calculation for quark-to-photon Collins
function, which is then used to estimate the Collins asymmetry for
fragmentation photons. We find that the Collins asymmetry for fragmentation
photons is very small, thus the single transverse spin asymmetry of prompt
photon production is mainly coming from the Sivers asymmetry in direct and
fragmentation photons. We make predictions for the prompt photon spin asymmetry
at RHIC energy, and emphasize the importance of such a measurement.
