  Assume $M$ is a closed 3-manifold whose universal covering is not $S^3$. We
show that the obstruction to extend the Ricci flow is the boundedness
$L^{3/2}$-norm of the scalar curvature $R(t)$, i.e, the Ricci flow can be
extended over time $T$ if and only if the $||R(t)||_{L^{3/2}}$ is uniformly
bounded for $0 \leq t < T$ . On the other hand, if the fundamental group of $M$
is finite and the $||R(t)||_{L^{\3/2}}$ is bounded for all time under the Ricci
flow, then $M$ is diffeomorphic to a 3-dimensional spherical space-form.
