We propose in this work a fast numerical algorithm for solving the equation of
radiative transfer (ERT) in isotropic media. The algorithm has two steps. In the first
step, we derive an integral equation for the angularly averaged ERT solution by taking
advantage of the isotropy of the scattering kernel, and solve the integral equation
with a fast multipole method (FMM). In the second step, we solve a scattering-free
transport equation to recover the original ERT solution. Numerical simulations are
presented to demonstrate the performance of the algorithm for both homogeneous and
inhomogeneous media.