  Let $\lambda:[0,+\infty)\mapsto\mathbb{R}$ be the driving function of a
chordal Loewner process. In this paper we find new conditions on $\lambda$
which imply that the process is generated by a simple curve. This result
improves former one by Lind ,Marshall and Rhode, and it particular gives new
results about the case $\lambda(t)=cW_b(t)$, $W_b$ being a H\"{o}lder-$1/2$
Weierstrass function. In the second part we find new conditions on $\lambda$
implying that the process is generated by a curve. The main tool here is a
duality relation between the real part and the imaginary part of the Loewner
equation.
