  We study the differentiability of Bessel flow $\rho : x \to \rho ^x_t$, where
$(\rho ^x_t)_{t\geq 0}$ is BES $^x(\delta $) process of dimension $\delta >1$
starting from $x$. For $\delta \geq 2$ we prove the existence of bicontinuous
derivatives in P-a.s. sense at $x\geq 0$ and we study the asymptotic behaviour
of the derivatives at $x=0$. For $1< \delta <2$ we prove the existence of a
modification of Bessel flow having derivatives in probability sense at $x\geq
0$. We study the asymptotic behaviour of the derivatives at $t=\tau_0(x)$ where
$\tau_0(x)$ is the first zero of $(\rho ^x_t)_{t\geq 0}$.
