  The detection of the delayed emission in X-ray, optical and radio band, i.e.
the afterglow of $\gamma$-ray bursts (GRBs), suggestes that the sources of GRBs
are likely to be at cosmological distances. Here we explore the interaction of
a relativistic shell with a uniform interstellar medium (ISM) and obtain the
exact solution of the evolution of $\gamma$-ray burst remnants, including the
radiative losses. We show that in general the evolution of bulk Lorentz factor
$\gamma$ satisfies $\gamma \propto t^{-\alpha_{t}}$ when $\gamma \gg 1$, here
$\alpha_{t}$ is mainly in the range $9/22 \sim 3/8$, the latter corresponds to
adiabatic expansion. So it is clear that adiabatic expansion is a good
approximation even when radiative loss is considered. However, in fact,
$\alpha_{t}$ is slightly larger than 3/8, which may have some effects on a
detailed data analysis. Synchrotron-self-absorption is also calculated and it
is demonstrated that the radio emission may become optically thin during the
afterglow. Our solution can also apply to the nonrelativistic case ($\gamma
\sim 1$), at that time the observed flux decrease more rapidly than that in the
relativistic case.
