  Quantum turbulence accompanying thermal counterflow in superfluid $^4$He was
recently visualized by the Maryland group, using micron-sized tracer particles
of solid hydrogen (J. Phys. Soc. Jpn. {\bf 77}, 111007 (2008)) . In order to
understand the observations we formulate the coupled dynamics of fine particles
and quantized vortices, in the presence of a relative motion of the normal and
superfluid components. Numerical simulations based on this formulation are
shown to agree reasonably well with experimental observations of the velocity
distributions of the tracer particles in thermal counterflow.
