  We consider the simplest quartic number fields $\mathbb{K}_m$ defined by the
irreducible quartic polynomials $$x^4-mx^3-6x^2+mx+1,$$ where $m$ runs over the
positive rational integers such that the odd part of $m^2+16$ is squarefree. In
this paper, we study the common index divisor $I(\mathbb K_m)$ and determine
explicitly the prime ideal decomposition for any prime number in any simplest
quartic number fields $\mathbb{K}_m$. On the other hand, we establish an
asymptotic formula for the number of simplest quartic fields with discriminant
$\leq x$ and given index.
