We call $K$ a $(1,1)$-knot in $M$ if $M$ is a union of two solid tori $V_1$ and $V_2$ glued along their boundary tori $partial V_1$ and $partial V_2$ and if $K$ intersects each solid torus $V_i$ in a trivial arc $t_i$ for $i=1$ and $2$. Note that every $(1,1)$-knot is a tunnel number one knot. In this article, we determine when a tunnel number one knot is a $(1,1)$-knot. In other words, we show that any tunnel number one knot with bridge number 3 is a $(1,1)$-knot

Keywords: tunnel number one knot, (1, 1)-decomposition, thin position