For a random quasi-abelian code of rate $r$, it is shown that the GV-bound is a threshold point: if $r$ is less than the GV-bound at $δ$, then the probability of the relative distance of the random code being greater than $δ$ is almost 1; whereas, if $r$ is bigger than the GV-bound at $δ$, then the probability is almost 0. As a consequence, there exist many asymptotically good quasi-abelian codes with any parameters attaining the GV-bound.